diff --git a/lib/src/error.rs b/lib/src/error.rs
index 88b7d7c..e583640 100644
--- a/lib/src/error.rs
+++ b/lib/src/error.rs
@@ -219,6 +219,31 @@ pub enum RuntimeError {
 	#[diagnostic(code(watchexec::runtime::clearscreen))]
 	Clearscreen(#[from] clearscreen::Error),
 
+	/// Error received when parsing a glob from an [`IgnoreFile`](crate::ignore::files::IgnoreFile) fails.
+	#[error("cannot parse glob from ignore '{file}': {err}")]
+	#[diagnostic(code(watchexec::runtime::ignore_file_glob))]
+	IgnoreFileGlob {
+		/// The path to the erroring ignore file.
+		file: PathBuf,
+
+		/// The underlying error.
+		#[source]
+		err: ignore::Error,
+		// TODO: extract glob error into diagnostic
+	},
+
+	/// Error received when an [`IgnoreFile`](crate::ignore::files::IgnoreFile) cannot be read.
+	#[error("cannot read ignore '{file}': {err}")]
+	#[diagnostic(code(watchexec::runtime::ignore_file_read))]
+	IgnoreFileRead {
+		/// The path to the erroring ignore file.
+		file: PathBuf,
+
+		/// The underlying error.
+		#[source]
+		err: std::io::Error,
+	},
+
 	/// Error emitted by a [`Filterer`](crate::filter::Filterer).
 	///
 	/// With built-in filterers this will probably be a dynbox of
diff --git a/lib/src/ignore.rs b/lib/src/ignore.rs
index 9e853c9..e17214f 100644
--- a/lib/src/ignore.rs
+++ b/lib/src/ignore.rs
@@ -1,3 +1,251 @@
 //! Ignore files: find them, parse them, interpret them.
 
+use std::path::{Path, PathBuf};
+
+use futures::stream::{FuturesUnordered, StreamExt};
+use ignore::{
+	gitignore::{Gitignore, GitignoreBuilder},
+	Match,
+};
+use tokio::fs::read_to_string;
+use tracing::{trace, trace_span};
+
+use crate::{
+	error::RuntimeError,
+	event::{Event, FileType},
+	filter::Filterer,
+};
+
 pub mod files;
+
+/// A path-only filterer dedicated to ignore files.
+///
+/// This reads and compiles ignore files, and should be used for handling ignore files. It's created
+/// with a project origin and a list of ignore files, and new ignore files can be added later
+/// (unless [`finish`](Filter::finish()) is called).
+///
+/// It implements [`Filterer`] so it can be used directly in another filterer; it is not designed to
+/// be used as a standalone filterer.
+#[derive(Debug)]
+pub struct Filter {
+	origin: PathBuf,
+	builder: Option<GitignoreBuilder>,
+	compiled: Gitignore,
+}
+
+impl Filter {
+	/// Read ignore files from disk and load them for filtering.
+	pub async fn new(
+		origin: impl AsRef<Path>,
+		files: &[files::IgnoreFile],
+	) -> Result<Self, RuntimeError> {
+		let (files_contents, errors): (Vec<_>, Vec<_>) = files
+			.iter()
+			.map(|file| async move {
+				trace!(?file, "loading ignore file");
+				let content = read_to_string(&file.path).await.map_err(|err| {
+					RuntimeError::IgnoreFileRead {
+						file: file.path.clone(),
+						err,
+					}
+				})?;
+				Ok((file.clone(), content))
+			})
+			.collect::<FuturesUnordered<_>>()
+			.collect::<Vec<_>>()
+			.await
+			.into_iter()
+			.map(|res| match res {
+				Ok(o) => (Some(o), None),
+				Err(e) => (None, Some(e)),
+			})
+			.unzip();
+
+		let errors: Vec<RuntimeError> = errors.into_iter().flatten().collect();
+		if !errors.is_empty() {
+			return Err(RuntimeError::Set(errors));
+		}
+
+		let origin = origin.as_ref();
+		let mut builder = GitignoreBuilder::new(origin);
+		for (file, content) in files_contents.into_iter().flatten() {
+			let _span = trace_span!("loading ignore file", ?file).entered();
+			for line in content.lines() {
+				if line.is_empty() || line.starts_with('#') {
+					continue;
+				}
+
+				trace!(?line, "adding ignore line");
+				builder
+					.add_line(file.applies_in.clone(), line)
+					.map_err(|err| RuntimeError::IgnoreFileGlob {
+						file: file.path.clone(),
+						err,
+					})?;
+			}
+		}
+
+		trace!("compiling globset");
+		let compiled = builder
+			.build()
+			.map_err(|err| RuntimeError::IgnoreFileGlob {
+				file: "set of ignores".into(),
+				err,
+			})?;
+
+		trace!(
+			files=%files.len(),
+			ignores=%compiled.num_ignores(),
+			allows=%compiled.num_whitelists(),
+			"ignore files loaded and compiled",
+		);
+
+		Ok(Self {
+			origin: origin.to_owned(),
+			builder: Some(builder),
+			compiled,
+		})
+	}
+
+	/// Deletes the internal builder, to save memory.
+	///
+	/// This makes it impossible to add new ignore files without re-compiling the whole set.
+	pub fn finish(&mut self) {
+		self.builder = None;
+	}
+
+	/// Reads and adds an ignore file, if the builder is available.
+	///
+	/// Does nothing silently otherwise.
+	pub async fn add_file(&mut self, file: &files::IgnoreFile) -> Result<(), RuntimeError> {
+		if let Some(ref mut builder) = self.builder {
+			trace!(?file, "reading ignore file");
+			let content =
+				read_to_string(&file.path)
+					.await
+					.map_err(|err| RuntimeError::IgnoreFileRead {
+						file: file.path.clone(),
+						err,
+					})?;
+
+			let _span = trace_span!("loading ignore file", ?file).entered();
+			for line in content.lines() {
+				if line.is_empty() || line.starts_with('#') {
+					continue;
+				}
+
+				trace!(?line, "adding ignore line");
+				builder
+					.add_line(file.applies_in.clone(), line)
+					.map_err(|err| RuntimeError::IgnoreFileGlob {
+						file: file.path.clone(),
+						err,
+					})?;
+			}
+
+			let pre_ignores = self.compiled.num_ignores();
+			let pre_allows = self.compiled.num_whitelists();
+
+			trace!("recompiling globset");
+			let recompiled = builder
+				.build()
+				.map_err(|err| RuntimeError::IgnoreFileGlob {
+					file: file.path.clone(),
+					err,
+				})?;
+
+			trace!(
+				new_ignores=%(recompiled.num_ignores() - pre_ignores),
+				new_allows=%(recompiled.num_whitelists() - pre_allows),
+				"ignore file loaded and set recompiled",
+			);
+			self.compiled = recompiled;
+		}
+
+		Ok(())
+	}
+
+	/// Check a particular folder path against the ignore set.
+	///
+	/// Returns `false` if the folder should be ignored.
+	///
+	/// Note that this is a slightly different implementation than the [`Filterer`] trait, as the
+	/// latter handles events with multiple associated paths.
+	pub fn check_dir(&self, path: &Path) -> bool {
+		let _span = trace_span!("check_dir", ?path).entered();
+
+		trace!("checking against compiled ignore files");
+		match if path.strip_prefix(&self.origin).is_ok() {
+			trace!("checking against path or parents");
+			self.compiled.matched_path_or_any_parents(path, true)
+		} else {
+			trace!("checking against path only");
+			self.compiled.matched(path, true)
+		} {
+			Match::None => {
+				trace!("no match (pass)");
+				true
+			}
+			Match::Ignore(glob) => {
+				if glob.from().map_or(true, |f| path.strip_prefix(f).is_ok()) {
+					trace!(?glob, "positive match (fail)");
+					false
+				} else {
+					trace!(?glob, "positive match, but not in scope (pass)");
+					true
+				}
+			}
+			Match::Whitelist(glob) => {
+				trace!(?glob, "negative match (pass)");
+				true
+			}
+		}
+	}
+}
+
+impl Filterer for Filter {
+	/// Filter an event.
+	///
+	/// This implementation never errors. It returns `Ok(false)` if the event is ignored according
+	/// to the ignore files, and `Ok(true)` otherwise.
+	fn check_event(&self, event: &Event) -> Result<bool, RuntimeError> {
+		let _span = trace_span!("filterer_check").entered();
+		let mut pass = true;
+
+		for (path, file_type) in event.paths() {
+			let _span = trace_span!("path", ?path).entered();
+			let is_dir = file_type
+				.map(|t| matches!(t, FileType::Dir))
+				.unwrap_or(false);
+
+			trace!("checking against compiled ignore files");
+			match if path.strip_prefix(&self.origin).is_ok() {
+				trace!("checking against path or parents");
+				self.compiled.matched_path_or_any_parents(path, is_dir)
+			} else {
+				trace!("checking against path only");
+				self.compiled.matched(path, is_dir)
+			} {
+				Match::None => {
+					trace!("no match (pass)");
+					pass &= true;
+				}
+				Match::Ignore(glob) => {
+					if glob.from().map_or(true, |f| path.strip_prefix(f).is_ok()) {
+						trace!(?glob, "positive match (fail)");
+						pass &= false;
+					} else {
+						trace!(?glob, "positive match, but not in scope (ignore)");
+					}
+				}
+				Match::Whitelist(glob) => {
+					trace!(?glob, "negative match (pass)");
+					pass = true;
+				}
+			}
+		}
+
+		trace!(?pass, "verdict");
+		Ok(pass)
+	}
+}