diff --git a/solitaire_data/src/lib.rs b/solitaire_data/src/lib.rs
index 68a144a..938bee5 100644
--- a/solitaire_data/src/lib.rs
+++ b/solitaire_data/src/lib.rs
@@ -45,3 +45,9 @@ pub mod achievements;
 pub use achievements::{
     achievements_file_path, load_achievements_from, save_achievements_to, AchievementRecord,
 };
+
+pub mod progress;
+pub use progress::{
+    level_for_xp, load_progress_from, progress_file_path, save_progress_to, xp_for_win,
+    PlayerProgress,
+};
diff --git a/solitaire_data/src/progress.rs b/solitaire_data/src/progress.rs
new file mode 100644
index 0000000..23e7e41
--- /dev/null
+++ b/solitaire_data/src/progress.rs
@@ -0,0 +1,246 @@
+//! Player progression — XP, level, unlocks, daily/weekly progress.
+//!
+//! Persisted to `progress.json` next to `stats.json` and `achievements.json`.
+
+use std::collections::HashMap;
+use std::fs;
+use std::io;
+use std::path::{Path, PathBuf};
+
+use chrono::{DateTime, NaiveDate, Utc};
+use serde::{Deserialize, Serialize};
+
+const APP_DIR_NAME: &str = "solitaire_quest";
+const FILE_NAME: &str = "progress.json";
+
+/// XP-to-level lookup. Matches ARCHITECTURE.md §13.
+///
+/// Levels 1–10: `level = floor(total_xp / 500)`
+/// Levels 11+:  `level = 10 + floor((total_xp - 5_000) / 1_000)`
+pub fn level_for_xp(xp: u64) -> u32 {
+    if xp < 5_000 {
+        (xp / 500) as u32
+    } else {
+        10 + ((xp - 5_000) / 1_000) as u32
+    }
+}
+
+/// XP awarded for winning a game.
+///
+/// Base 50 + scaled fast-win bonus (10..=50 for sub-2-minute wins) + 25 if
+/// the player did not use undo.
+pub fn xp_for_win(time_seconds: u64, used_undo: bool) -> u64 {
+    let base: u64 = 50;
+    let speed_bonus: u64 = if time_seconds >= 120 {
+        0
+    } else {
+        // Linearly scale 50 → 10 across 0..=120 seconds.
+        // 0s → 50, 60s → 30, 120s → 10.
+        let scaled = 50_u64.saturating_sub(time_seconds.saturating_mul(40) / 120);
+        scaled.max(10)
+    };
+    let no_undo_bonus: u64 = if used_undo { 0 } else { 25 };
+    base + speed_bonus + no_undo_bonus
+}
+
+/// Persisted player progression state.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct PlayerProgress {
+    pub total_xp: u64,
+    pub level: u32,
+    pub daily_challenge_last_completed: Option<NaiveDate>,
+    pub daily_challenge_streak: u32,
+    pub weekly_goal_progress: HashMap<String, u32>,
+    pub unlocked_card_backs: Vec<usize>,
+    pub unlocked_backgrounds: Vec<usize>,
+    pub last_modified: DateTime<Utc>,
+}
+
+impl Default for PlayerProgress {
+    fn default() -> Self {
+        Self {
+            total_xp: 0,
+            level: 0,
+            daily_challenge_last_completed: None,
+            daily_challenge_streak: 0,
+            weekly_goal_progress: HashMap::new(),
+            unlocked_card_backs: vec![0],   // back #0 always available
+            unlocked_backgrounds: vec![0],  // background #0 always available
+            last_modified: DateTime::UNIX_EPOCH,
+        }
+    }
+}
+
+impl PlayerProgress {
+    /// Add XP and recompute level. Returns the previous level so callers can
+    /// detect level-up events.
+    pub fn add_xp(&mut self, amount: u64) -> u32 {
+        let prev_level = self.level;
+        self.total_xp = self.total_xp.saturating_add(amount);
+        self.level = level_for_xp(self.total_xp);
+        self.last_modified = Utc::now();
+        prev_level
+    }
+
+    /// `true` if a level-up just occurred (current level > `prev_level`).
+    pub fn leveled_up_from(&self, prev_level: u32) -> bool {
+        self.level > prev_level
+    }
+}
+
+/// Platform-specific default path for `progress.json`.
+pub fn progress_file_path() -> Option<PathBuf> {
+    dirs::data_dir().map(|d| d.join(APP_DIR_NAME).join(FILE_NAME))
+}
+
+/// Load progress from an explicit path. Returns `default()` if missing/corrupt.
+pub fn load_progress_from(path: &Path) -> PlayerProgress {
+    let Ok(data) = fs::read(path) else {
+        return PlayerProgress::default();
+    };
+    serde_json::from_slice(&data).unwrap_or_default()
+}
+
+/// Save progress to an explicit path using an atomic write.
+pub fn save_progress_to(path: &Path, progress: &PlayerProgress) -> io::Result<()> {
+    if let Some(parent) = path.parent() {
+        fs::create_dir_all(parent)?;
+    }
+    let json = serde_json::to_string_pretty(progress).map_err(io::Error::other)?;
+    let tmp = path.with_extension("json.tmp");
+    fs::write(&tmp, json.as_bytes())?;
+    fs::rename(&tmp, path)?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::env;
+
+    fn tmp_path(name: &str) -> PathBuf {
+        env::temp_dir().join(format!("solitaire_progress_test_{name}.json"))
+    }
+
+    // --- Level formula ---
+
+    #[test]
+    fn level_for_xp_at_breakpoints() {
+        assert_eq!(level_for_xp(0), 0);
+        assert_eq!(level_for_xp(499), 0);
+        assert_eq!(level_for_xp(500), 1);
+        assert_eq!(level_for_xp(4_999), 9);
+        assert_eq!(level_for_xp(5_000), 10);
+        assert_eq!(level_for_xp(5_999), 10);
+        assert_eq!(level_for_xp(6_000), 11);
+        assert_eq!(level_for_xp(15_000), 20);
+    }
+
+    // --- XP-for-win formula ---
+
+    #[test]
+    fn xp_for_slow_win_with_undo_is_just_base() {
+        assert_eq!(xp_for_win(300, true), 50);
+    }
+
+    #[test]
+    fn xp_for_no_undo_win_adds_25() {
+        assert_eq!(xp_for_win(300, false), 75);
+    }
+
+    #[test]
+    fn xp_for_instant_win_includes_max_speed_bonus() {
+        // base 50 + speed 50 = 100 with undo, +25 without
+        assert_eq!(xp_for_win(0, true), 100);
+        assert_eq!(xp_for_win(0, false), 125);
+    }
+
+    #[test]
+    fn xp_speed_bonus_scales_linearly_to_120s() {
+        // At 60s: 50 - (60*40/120) = 50 - 20 = 30
+        assert_eq!(xp_for_win(60, true), 50 + 30);
+        // At 119s: 50 - (119*40/120) = 50 - 39 = 11, but floored at 10
+        assert!(xp_for_win(119, true) >= 60);
+    }
+
+    #[test]
+    fn xp_no_speed_bonus_at_or_above_120s() {
+        assert_eq!(xp_for_win(120, true), 50);
+        assert_eq!(xp_for_win(180, true), 50);
+    }
+
+    // --- PlayerProgress.add_xp ---
+
+    #[test]
+    fn add_xp_returns_previous_level_and_recomputes() {
+        let mut p = PlayerProgress::default();
+        let prev = p.add_xp(500);
+        assert_eq!(prev, 0);
+        assert_eq!(p.total_xp, 500);
+        assert_eq!(p.level, 1);
+    }
+
+    #[test]
+    fn level_up_detection_works() {
+        let mut p = PlayerProgress::default();
+        let prev = p.add_xp(450);
+        assert!(!p.leveled_up_from(prev), "no level change at 450 xp");
+        let prev = p.add_xp(60);
+        assert!(p.leveled_up_from(prev), "0 → 1 at 510 xp");
+    }
+
+    #[test]
+    fn add_xp_saturates_on_overflow() {
+        let mut p = PlayerProgress::default();
+        p.total_xp = u64::MAX - 5;
+        p.add_xp(100);
+        assert_eq!(p.total_xp, u64::MAX);
+    }
+
+    #[test]
+    fn default_unlocks_include_first_card_back_and_background() {
+        let p = PlayerProgress::default();
+        assert!(p.unlocked_card_backs.contains(&0));
+        assert!(p.unlocked_backgrounds.contains(&0));
+    }
+
+    // --- Persistence ---
+
+    #[test]
+    fn round_trip_save_and_load() {
+        let path = tmp_path("round_trip");
+        let _ = fs::remove_file(&path);
+
+        let mut p = PlayerProgress::default();
+        p.add_xp(1234);
+        p.unlocked_card_backs.push(2);
+        save_progress_to(&path, &p).expect("save");
+        let loaded = load_progress_from(&path);
+        assert_eq!(loaded.total_xp, 1234);
+        assert_eq!(loaded.level, p.level);
+        assert!(loaded.unlocked_card_backs.contains(&2));
+    }
+
+    #[test]
+    fn load_from_missing_file_returns_default() {
+        let path = tmp_path("missing_xyz");
+        let _ = fs::remove_file(&path);
+        let p = load_progress_from(&path);
+        assert_eq!(p, PlayerProgress::default());
+    }
+
+    #[test]
+    fn load_from_corrupt_file_returns_default() {
+        let path = tmp_path("corrupt");
+        fs::write(&path, b"garbage").expect("write");
+        let p = load_progress_from(&path);
+        assert_eq!(p, PlayerProgress::default());
+    }
+
+    #[test]
+    fn save_cleans_up_tmp_file() {
+        let path = tmp_path("atomic");
+        save_progress_to(&path, &PlayerProgress::default()).expect("save");
+        assert!(!path.with_extension("json.tmp").exists());
+    }
+}