diff --git a/src/board_game_recommender/baseline.py b/src/board_game_recommender/baseline.py
new file mode 100644
index 0000000..d3481f3
--- /dev/null
+++ b/src/board_game_recommender/baseline.py
@@ -0,0 +1,329 @@
+"""Baseline recommender models."""
+
+from __future__ import annotations
+
+from collections import defaultdict
+from typing import TYPE_CHECKING, cast
+
+import numpy as np
+import polars as pl
+
+from board_game_recommender.abc import BaseGamesRecommender
+
+if TYPE_CHECKING:
+    from collections.abc import Iterable
+    from pathlib import Path
+    from typing import Any, Self
+
+
+def dataframe_from_scores(
+    *,
+    users: Iterable[str],
+    games: Iterable[Any],
+    scores: np.ndarray,  # shape: (users, games)
+) -> pl.DataFrame:
+    """Creates a Pandas DataFrame out of raw recommendation scores."""
+
+    users = tuple(users)
+    games = tuple(games)
+
+    rank_cols = (
+        pl.col(f"{column}_score")
+        .rank(method="min", descending=True)
+        .alias(f"{column}_rank")
+        for column in users
+    )
+
+    result = (
+        pl.DataFrame(
+            data=scores.T,
+            schema=[f"{column}_score" for column in users],
+        )
+        .lazy()
+        .with_columns(
+            pl.Series("index", games),
+            *rank_cols,
+        )
+    )
+
+    if len(users) == 1:
+        result = result.sort(f"{users[0]}_rank")
+
+    return result.collect()
+
+
+class RandomGamesRecommender(BaseGamesRecommender):
+    """Random recommender."""
+
+    def __init__(self) -> None:
+        self.rng = np.random.default_rng()
+
+    @property
+    def known_games(self) -> frozenset[int]:
+        return frozenset()
+
+    @property
+    def rated_games(self) -> frozenset[int]:
+        return frozenset()
+
+    @property
+    def known_users(self) -> frozenset[str]:
+        return frozenset()
+
+    def _recommendation_scores(self, users: int, games: int) -> np.ndarray:
+        """Random scores."""
+        return self.rng.random((users, games))
+
+    def recommend(  # type: ignore[override]
+        self,
+        users: Iterable[str],
+        games: Iterable[int],
+        **kwargs,  # noqa: ARG002
+    ) -> pl.DataFrame:
+        """Random recommendations for certain users."""
+
+        users = list(users)
+        games = list(games)
+        scores = self._recommendation_scores(users=len(users), games=len(games))
+
+        return dataframe_from_scores(users=users, games=games, scores=scores)
+
+    def recommend_as_numpy(
+        self,
+        users: Iterable[str],
+        games: Iterable[int],
+    ) -> np.ndarray:
+        """Random recommendations for certain users and games as a numpy array."""
+        users = list(users)
+        games = list(games)
+        return self._recommendation_scores(users=len(users), games=len(games))
+
+    def recommend_group(  # type: ignore[override]
+        self,
+        users: Iterable[str],  # noqa: ARG002
+        games: Iterable[int],
+        **kwargs,  # noqa: ARG002
+    ) -> pl.DataFrame:
+        """Random recommendations for a group of users."""
+
+        games = list(games)
+        scores = self._recommendation_scores(users=1, games=len(games))
+
+        return dataframe_from_scores(users=["_all"], games=games, scores=scores)
+
+    def recommend_group_as_numpy(
+        self,
+        users: Iterable[str],  # noqa: ARG002
+        games: Iterable[int],
+    ) -> np.ndarray:
+        """Random recommendations for a group of users and games as a numpy array."""
+        games = list(games)
+        return self._recommendation_scores(users=1, games=len(games))
+
+    def recommend_similar(self, games: Iterable[int], **kwargs) -> pl.DataFrame:
+        raise NotImplementedError
+
+    def similar_games(self, games: Iterable[int], **kwargs) -> pl.DataFrame:
+        raise NotImplementedError
+
+
+class PopularGamesRecommender(BaseGamesRecommender):
+    """Popular games recommender."""
+
+    id_field: str = "bgg_id"
+    user_id_field: str = "bgg_user_name"
+    rating_id_field: str = "bgg_user_rating"
+
+    scores: dict[int, float]
+    raw_scores: np.ndarray
+    default_value: float
+    game_ids: tuple[int, ...]
+
+    _known_games: frozenset[int] | None = None
+
+    def __init__(
+        self,
+        game_ids: Iterable[int],
+        scores: np.ndarray,
+        default_value: float | None = None,
+    ) -> None:
+        self.default_value = (
+            default_value if default_value is not None else scores.mean()
+        )
+        self.raw_scores = scores
+        self.game_ids = tuple(game_ids)
+        self.scores = defaultdict(
+            self.default_factory,
+            zip(self.game_ids, self.raw_scores),
+        )
+
+    @classmethod
+    def train(cls, ratings: pl.DataFrame) -> Self:
+        """Train the recommender from ratings data."""
+        raise NotImplementedError
+
+    @classmethod
+    def train_from_csv(cls, ratings_file: Path | str) -> Self:
+        """Train the recommender from a ratings file in CSV format."""
+        ratings = pl.read_csv(ratings_file)
+        return cls.train(
+            ratings.select(cls.id_field, cls.user_id_field, cls.rating_id_field),
+        )
+
+    @classmethod
+    def train_from_json_lines(cls, ratings_file: Path | str) -> Self:
+        """Train the recommender from a ratings file in JSON lines format."""
+        ratings = pl.read_ndjson(ratings_file)
+        return cls.train(
+            ratings.select(cls.id_field, cls.user_id_field, cls.rating_id_field),
+        )
+
+    @property
+    def known_games(self) -> frozenset[int]:
+        if self._known_games is not None:
+            return self._known_games
+        self._known_games = frozenset(self.game_ids)
+        return self._known_games
+
+    @property
+    def rated_games(self) -> frozenset[int]:
+        return self.known_games
+
+    @property
+    def num_games(self) -> int:
+        return len(self.game_ids)
+
+    @property
+    def known_users(self) -> frozenset[str]:
+        return frozenset()
+
+    def default_factory(self) -> float:
+        """Default value for unknown games."""
+        return self.default_value
+
+    def _recommendation_scores(
+        self,
+        users: int,
+        games: list[int] | None = None,
+    ) -> np.ndarray:
+        """Popularity scores."""
+        scores = (
+            np.array([self.scores[game_id] for game_id in games])
+            if games
+            else self.raw_scores
+        )
+        return np.tile(scores, [users, 1])
+
+    def recommend(
+        self,
+        users: Iterable[str],
+        **kwargs,  # noqa: ARG002
+    ) -> pl.DataFrame:
+        """Popular recommendations for certain users."""
+        users = list(users)
+        scores = self._recommendation_scores(users=len(users))
+        return dataframe_from_scores(users=users, games=self.game_ids, scores=scores)
+
+    def recommend_as_numpy(
+        self,
+        users: Iterable[str],
+        games: Iterable[int],
+    ) -> np.ndarray:
+        """Popular recommendations for certain users and games as a numpy array."""
+        users = list(users)
+        games = list(games)
+        return self._recommendation_scores(users=len(users), games=games)
+
+    def recommend_group(
+        self,
+        users: Iterable[str],  # noqa: ARG002
+        **kwargs,  # noqa: ARG002
+    ) -> pl.DataFrame:
+        """Popular recommendations for a group of users."""
+        scores = self._recommendation_scores(users=1)
+        return dataframe_from_scores(users=["_all"], games=self.game_ids, scores=scores)
+
+    def recommend_group_as_numpy(
+        self,
+        users: Iterable[str],  # noqa: ARG002
+        games: Iterable[int],
+    ) -> np.ndarray:
+        """Popular recommendations for a group of users and games as a numpy array."""
+        games = list(games)
+        return self._recommendation_scores(users=1, games=games)
+
+    def recommend_similar(
+        self,
+        games: Iterable[int],
+        **kwargs,
+    ) -> pl.DataFrame:
+        raise NotImplementedError
+
+    def similar_games(
+        self,
+        games: Iterable[int],
+        **kwargs,
+    ) -> pl.DataFrame:
+        raise NotImplementedError
+
+
+class PopularMeanGamesRecommender(PopularGamesRecommender):
+    """Recommend games by their mean rating score."""
+
+    @classmethod
+    def train(cls, ratings: pl.DataFrame) -> Self:
+        data = ratings.group_by(cls.id_field).agg(
+            mean=pl.col(cls.rating_id_field).mean(),
+        )
+        return cls(
+            game_ids=data[cls.id_field],
+            scores=data["mean"].to_numpy(),
+            default_value=cast(float, ratings[cls.rating_id_field].mean()),
+        )
+
+
+class PopularBayesianGamesRecommender(PopularGamesRecommender):
+    """Recommend games by their Bayesian average rating score."""
+
+    ratings_per_dummy: float = 10_000
+    dummy_rating: float | None = 5.5
+
+    @classmethod
+    def train(cls, ratings: pl.DataFrame) -> Self:
+        num_dummies = len(ratings) / cls.ratings_per_dummy
+        dummy_rating = cast(
+            float,
+            ratings[cls.rating_id_field].mean()
+            if cls.dummy_rating is None
+            else cls.dummy_rating,
+        )
+
+        stats = (
+            ratings.group_by(cls.id_field)
+            .agg(
+                mean=pl.col(cls.rating_id_field).mean(),
+                size=pl.len(),
+            )
+            .with_columns(
+                bayes=pl.col("mean") * pl.col("size")
+                + dummy_rating * num_dummies / (pl.col("size") + num_dummies),
+            )
+        )
+
+        return cls(
+            game_ids=stats[cls.id_field],
+            scores=stats["bayes"].to_numpy(),
+            default_value=dummy_rating,
+        )
+
+
+class PopularNumRatingsGamesRecommender(PopularGamesRecommender):
+    """Recommend games by their number of ratings."""
+
+    @classmethod
+    def train(cls, ratings: pl.DataFrame) -> Self:
+        data = ratings.group_by(cls.id_field).agg(count=pl.len())
+        return cls(
+            game_ids=data[cls.id_field],
+            scores=data["count"].to_numpy(),
+        )