We review known results and derive some new ones about the mean free path, Kolmogorov-Sinai entropy, and Lyapunov exponents
for billiard-type dynamical systems. We focus on exact and asymptotic formulas for these quantities. The dynamical systems
covered in this paper include the priodic Lorentz gas, the stadium and its modifications, and the gas of hard balls. Some
open questions and numerical observations are discussed.
Key words Billiards - hard balls - Lorentz gas - entropy - mean free path - Lyapunov exponents