This book shows how densities arise in simple deterministic systems.

1. Introduction; 2. The toolbox; 3. Markov and Frobenius-Perron operators; 4. Studying chaos with densities; 5. The asymptotic properties of densities; 6. The behaviour of transformations on intervals and manifolds; 7. Continuous time systems: an introduction; 8. Discrete time processes embedded in continuous time systems; 9. Entropy; 10. Stochastic perturbation of discrete time systems; 11. Stochastic perturbation of continuous time systems.