diff --git a/lib/rackstash.rb b/lib/rackstash.rb
index 4717a23..28987c2 100644
--- a/lib/rackstash.rb
+++ b/lib/rackstash.rb
@@ -47,9 +47,19 @@ module Rackstash
   FIELD_TAGS = 'tags'.freeze
   FIELD_TIMESTAMP = '@timestamp'.freeze
   FIELD_VERSION = '@version'.freeze
+
+  def self.severity_label(severity)
+    if severity.is_a?(Integer)
+      SEVERITY_LABELS[severity] || SEVERITY_LABELS.last
+    else
+      severity = SEVERITY_NAMES.fetch(severity.to_s.downcase, UNKNOWN)
+      SEVERITY_LABELS[severity]
+    end
+  end
 end
 
 require 'rackstash/logger'
 
-require 'rackstash/adapters/io'
 require 'rackstash/adapters/callable'
+require 'rackstash/adapters/file'
+require 'rackstash/adapters/io'
diff --git a/lib/rackstash/adapters/file.rb b/lib/rackstash/adapters/file.rb
new file mode 100644
index 0000000..ef8bb53
--- /dev/null
+++ b/lib/rackstash/adapters/file.rb
@@ -0,0 +1,173 @@
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'fileutils'
+require 'pathname'
+require 'thread'
+
+require 'rackstash/adapters/adapter'
+
+module Rackstash
+  module Adapters
+    # This log adapter allows to write logs to a file acessible on the local
+    # filesystem. We assume filesystem semantics of the usual local filesystems
+    # used on Linux, macOS, BSDs, or Windows. Here, we can ensure that even
+    # concurrent writes of multiple processes (e.g. multiple worker processes of
+    # an application server) don't produce interleaved log lines.
+    #
+    # When using a remote filesystem (e.g. NFS or most FUSE filesystems but not
+    # for SMB) it might be possible that concurrent log writes to the same file
+    # are interleaved on disk, resulting on probable log corruption. If this is
+    # a concern, you should make sure that only one log adapter of one process
+    # write to a log file at a time or (preferrably) write to a local file
+    # instead.
+    #
+    # When reading the log file, the reader might still see incomplete writes
+    # depending on the OS and filesystem. Since we are only writing complete
+    # lines, it should be safe to continue reading until you observe a newline
+    # (`\n`) character.
+    #
+    # Assuming you are creating the log adapter like this
+    #
+    #     Rackstash::Adapters::File.new('/var/log/rackstash/my_app.log')
+    #
+    # you can rotate the file with a config for the the standard
+    # [logrotate](https://github.com/logrotate/logrotate) utility like this:
+    #
+    #     /var/log/rackstash/my_app.log {
+    #       daily
+    #       rotate 30
+    #
+    #       # file might be missing if there were no writes that day
+    #       missingok
+    #       notifempty
+    #
+    #       # compress old logfiles but keep the newest rotate file uncompressed
+    #       # to still allow writes during rotation
+    #       compress
+    #       delaycompress
+    #     }
+    #
+    # Since the {File} adapter automatically reopens the logfile after the
+    # file was moved, you don't need to create the new file there nor should you
+    # use the (potentially destructive) `copytruncate` option of logrotate.
+    class File < Adapter
+      register_for ::String, ::Pathname
+
+      # @return [String] the absolute path to the log file
+      attr_reader :filename
+
+      # Create a new file adapter instance which writes logs to the log file
+      # specified in `filename`.
+      #
+      # We will always resolve `filename` to an absolute path once during
+      # initialization. When passing a relative path, it will be resolved
+      # according to the current working directory. See
+      # [`::File.expand_path`](https://ruby-doc.org/core/File.html#method-c-expand_path)
+      # for details.
+      #
+      # @param filename [String, Pathname] the path to the logfile
+      # @param auto_reopen [Boolean] set to `true` to automatically reopen the
+      #   log file (and potentially create a new one) if we detect that the
+      #   current log file was moved or deleted, e.g. due to an external
+      #   logrotate run
+      def initialize(filename, auto_reopen: true)
+        @filename = ::File.expand_path(filename).freeze
+        @auto_reopen = !!auto_reopen
+
+        @mutex = Mutex.new
+        open_file
+      end
+
+      # @return [Boolean] if `true`, the logfile will be automatically reopened
+      #   on write if it is (re-)moved on the filesystem
+      def auto_reopen?
+        @auto_reopen
+      end
+
+      # Write a single log line with a trailing newline character to the open
+      # file. If {#auto_reopen?} is `true`, we will reopen the file object
+      # before the write if we detect that the file was moved, e.g., from an
+      # external logrotate run.
+      #
+      # When writing the log line, ruby uses a single `fwrite(2)` syscall with
+      # `IO#write`. Since we are using unbuffered (sync) IO, concurrent writes
+      # to the file from multiple processes
+      # [are guaranteed](https://stackoverflow.com/a/35256561/421705) to be
+      # serialized by the kernel without overlap.
+      #
+      # @param log [#to_s] the encoded log event
+      # @return [nil]
+      def write_single(log)
+        @mutex.synchronize do
+          auto_reopen
+          @file.write normalize_line(log)
+        end
+        nil
+      end
+
+      # Close the file. After closing, no further writes are possible. Further
+      # attempts to {#write} will result in an exception being thrown.
+      #
+      # We will not automatically reopen a closed file on {#write}. You have to
+      # explicitly call {#reopen} in this case.
+      #
+      # @return [nil]
+      def close
+        @mutex.synchronize do
+          @file.close
+        end
+        nil
+      end
+
+      # Reopen the logfile. We will open the file located at the original
+      # {#filename} or create a new one if it does not exist.
+      #
+      # If the file can not be opened, an exception will be raised.
+      # @return [nil]
+      def reopen
+        @mutex.synchronize do
+          reopen_file
+        end
+        nil
+      end
+
+      private
+
+      # Reopen the log file if the original filename does not reference the
+      # opened file anymore (e.g. because it was moved or deleted)
+      def auto_reopen
+        return unless @auto_reopen
+
+        return if @file.closed?
+        return if ::File.identical?(@file, @filename)
+
+        reopen_file
+      end
+
+      def open_file
+        unless ::File.exist? ::File.dirname(@filename)
+          FileUtils.mkdir_p ::File.dirname(@filename)
+        end
+
+        file = ::File.open(
+          filename,
+          ::File::WRONLY | ::File::APPEND | ::File::CREAT,
+          external_encoding: Encoding::UTF_8
+        )
+        file.binmode
+        file.sync = true
+
+        @file = file
+        nil
+      end
+
+      def reopen_file
+        @file.close rescue nil
+        open_file
+      end
+    end
+  end
+end
diff --git a/spec/rackstash/adapters/file_spec.rb b/spec/rackstash/adapters/file_spec.rb
new file mode 100644
index 0000000..b89fab5
--- /dev/null
+++ b/spec/rackstash/adapters/file_spec.rb
@@ -0,0 +1,169 @@
+require 'spec_helper'
+require 'tempfile'
+require 'tmpdir'
+
+require 'rackstash/adapters/file'
+
+describe Rackstash::Adapters::File do
+  let!(:logfile) { Tempfile.new('') }
+
+  let(:adapter_args) { {} }
+  let(:adapter) { described_class.new(logfile.path, **adapter_args) }
+
+  after(:each) do
+    logfile.close
+    logfile.unlink
+  end
+
+  describe '#initialize' do
+    it 'accepts a String' do
+      expect(described_class.new(logfile.path).filename)
+        .to eql(logfile.path)
+        .and be_a String
+    end
+
+    it 'accepts a Pathname' do
+      expect(described_class.new(Pathname.new logfile.path).filename)
+        .to eql(logfile.path)
+        .and be_a String
+    end
+
+    it 'rejects non-IO objects' do
+      expect { described_class.new(nil) }.to raise_error TypeError
+      expect { described_class.new(Object.new) }.to raise_error TypeError
+      expect { described_class.new(23) }.to raise_error TypeError
+    end
+
+    it 'creates the file and leading directories' do
+      Dir.mktmpdir do |base|
+        expect(File.exist? File.join(base, 'dir')).to be false
+
+        adapter = described_class.new File.join(base, 'dir', 'sub', 'test.log')
+
+        expect(adapter.filename).to eql File.join(base, 'dir', 'sub', 'test.log')
+        expect(File.directory? File.join(base, 'dir')).to be true
+        expect(File.file? File.join(base, 'dir', 'sub', 'test.log')).to be true
+      end
+    end
+  end
+
+  describe '.default_encoder' do
+    it 'returns a JSON encoder' do
+      expect(adapter.default_encoder).to be_instance_of Rackstash::Encoders::JSON
+    end
+  end
+
+  describe '#close' do
+    it 'closes the IO object' do
+      adapter.close
+      expect { adapter.write('hello') }.to raise_error IOError
+    end
+  end
+
+  describe '#reopen' do
+    it 're-opens a closed file' do
+      adapter.close
+      adapter.reopen
+
+      expect { adapter.write('hello') }.not_to raise_error
+    end
+  end
+
+  describe '#write_single' do
+    it 'writes the log line to the file' do
+      adapter.write('a log line')
+      expect(logfile.tap(&:rewind).read).to eql "a log line\n"
+    end
+
+    it 'always writes a string' do
+      adapter.write([123, 'hello'])
+      expect(logfile.tap(&:rewind).read).to eql "[123, \"hello\"]\n"
+    end
+
+    it 'appends a trailing newline if necessary' do
+      adapter.write("a full line.\n")
+      expect(logfile.tap(&:rewind).read).to eql "a full line.\n"
+    end
+
+    context 'with auto_reopen: true' do
+      let(:adapter_args) { { auto_reopen: true } }
+
+      it 'reopens the file if moved' do
+        adapter.write('line1')
+        File.rename(logfile.path, "#{logfile.path}.orig")
+
+        adapter.write('line2')
+
+        expect(File.read("#{logfile.path}.orig")).to eql "line1\n"
+        expect(File.read(logfile.path)).to eql "line2\n"
+      end
+    end
+
+    context 'with auto_reopen: false' do
+      let(:adapter_args) { { auto_reopen: false } }
+
+      it 'does not reopen the logfile automatically' do
+        adapter.write('line1')
+        File.rename(logfile.path, "#{logfile.path}.orig")
+
+        adapter.write('line2')
+
+        expect(File.read("#{logfile.path}.orig")).to eql "line1\nline2\n"
+        expect(File.exist?(logfile.path)).to be false
+      end
+    end
+  end
+
+  context 'with concurrent processes' do
+    let(:workers) { 20 }
+    let(:lines_per_worker) { 50 }
+    let(:line_length) { 4096 }
+
+    def run_worker(worker_id)
+      filler = (worker_id + 65).chr
+      line = filler * line_length
+
+      adapter = described_class.new(logfile.path)
+      lines_per_worker.times do
+        adapter.write(line)
+      end
+    end
+
+    # This test was adapted from
+    # http://www.notthewizard.com/2014/06/17/are-files-appends-really-atomic/
+    it 'writes atomic log lines' do
+      if Concurrent.on_cruby?
+        worker_processes = workers.times.map { |worker_id|
+          Process.fork do
+            run_worker worker_id
+          end
+        }
+        worker_processes.each do |pid|
+          Process.wait(pid)
+        end
+      else
+        worker_threads = workers.times.map { |worker_id|
+          Thread.new do
+            run_worker worker_id
+          end
+        }
+        worker_threads.each do |thread|
+          thread.join
+        end
+      end
+
+      # Resulting file size is exactly as expected, i.e. no dropped logs
+      # Each line as a trailing newline character.
+      expect(logfile.size).to eql workers * lines_per_worker * (line_length + 1)
+
+      # All lines are written without any overlap
+      expect(File.new(logfile.path).each_line).to all match(/\A(.)\1{#{line_length - 1}}\n\z/)
+
+      # Ensure that not all lines are written sequentially by the same worker,
+      # i.e. there were concurrent writes by multiple workers.
+      expect(
+        File.new(logfile.path).each_line.each_cons(2).count { |l1, l2| l1.to_s[0] == l2.to_s[0] }
+      ).to be > workers
+    end
+  end
+end