diff --git a/src/api/msg.rs b/src/api/msg.rs
index a8100a9..cad0b12 100644
--- a/src/api/msg.rs
+++ b/src/api/msg.rs
@@ -3,6 +3,8 @@
 // The MessagePack format for these is a one-element map where the element's key is the enum name and its
 // value is a map of the enum's values
 
+mod window_rules;
+
 use crate::{
     layout::Layout,
     output::OutputName,
diff --git a/src/api/msg/window_rules.rs b/src/api/msg/window_rules.rs
new file mode 100644
index 0000000..3ad783c
--- /dev/null
+++ b/src/api/msg/window_rules.rs
@@ -0,0 +1,107 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+use std::num::NonZeroU32;
+
+use smithay::wayland::{compositor, shell::xdg::XdgToplevelSurfaceData};
+
+use crate::{
+    output::OutputName,
+    state::{State, WithState},
+    tag::TagId,
+    window::{window_state::FullscreenOrMaximized, WindowElement},
+};
+
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub enum WindowRuleCondition {
+    /// This condition is met when any of the conditions provided is met.
+    CondAny(Vec<WindowRuleCondition>),
+    /// This condition is met when all of the conditions provided are met.
+    CondAll(Vec<WindowRuleCondition>),
+    /// This condition is met when the class matches.
+    Class(String),
+    /// This condition is met when the title matches.
+    Title(String),
+    /// This condition is met when the tag matches.
+    Tag(TagId),
+}
+
+impl WindowRuleCondition {
+    /// RefCell Safety: This method uses RefCells on `window`.
+    pub fn is_met(&self, state: &State, window: &WindowElement) -> bool {
+        match self {
+            WindowRuleCondition::CondAny(conds) => {
+                conds.iter().any(|cond| Self::is_met(cond, state, window))
+            }
+            WindowRuleCondition::CondAll(conds) => {
+                conds.iter().all(|cond| Self::is_met(cond, state, window))
+            }
+            WindowRuleCondition::Class(class) => {
+                let Some(wl_surf) = window.wl_surface() else {
+                    return false;
+                };
+
+                let current_class = compositor::with_states(&wl_surf, |states| {
+                    states
+                        .data_map
+                        .get::<XdgToplevelSurfaceData>()
+                        .expect("XdgToplevelSurfaceData wasn't in surface's data map")
+                        .lock()
+                        .expect("Failed to lock Mutex<XdgToplevelSurfaceData>")
+                        .app_id
+                        .clone()
+                });
+
+                current_class.as_ref() == Some(class)
+            }
+            WindowRuleCondition::Title(title) => {
+                let Some(wl_surf) = window.wl_surface() else {
+                    return false;
+                };
+
+                let current_title = compositor::with_states(&wl_surf, |states| {
+                    states
+                        .data_map
+                        .get::<XdgToplevelSurfaceData>()
+                        .expect("XdgToplevelSurfaceData wasn't in surface's data map")
+                        .lock()
+                        .expect("Failed to lock Mutex<XdgToplevelSurfaceData>")
+                        .title
+                        .clone()
+                });
+
+                current_title.as_ref() == Some(title)
+            }
+            WindowRuleCondition::Tag(tag) => {
+                let Some(tag) = tag.tag(state) else {
+                    return false;
+                };
+
+                window.with_state(|state| state.tags.contains(&tag))
+            }
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub enum WindowRule {
+    /// Set the output the window will open on.
+    Output(OutputName),
+    /// Set the tags the output will have on open.
+    Tags(Vec<TagId>),
+    /// Set the window to floating or tiled on open.
+    FloatingOrTiled(FloatingOrTiled),
+    /// Set the window to fullscreen, maximized, or force it to neither.
+    FullscreenOrMaximized(FullscreenOrMaximized),
+    /// Set the window's initial size.
+    Size(NonZeroU32, NonZeroU32),
+    /// Set the window's initial location. If the window is tiled, it will snap to this position
+    /// when set to floating.
+    Location(i32, i32),
+}
+
+// TODO: just skip serializing fields on the other FloatingOrTiled
+#[derive(Debug, Copy, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+pub enum FloatingOrTiled {
+    Floating,
+    Tiled,
+}
diff --git a/src/window/window_state.rs b/src/window/window_state.rs
index ff81b47..9cf99d2 100644
--- a/src/window/window_state.rs
+++ b/src/window/window_state.rs
@@ -301,7 +301,7 @@ impl FloatingOrTiled {
     }
 }
 
-#[derive(Debug, Clone, Copy, serde::Serialize, serde::Deserialize)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum FullscreenOrMaximized {
     Neither,
     Fullscreen,