In the past decade a number of fluctuation theorems have been obtained for nonequilibrium particle systems and fluids whose dynamics is described by deterministic or stochastic processes. These theorems aim, among other things, at extending Onsager's theory of irreversible thermodynamics, beyond the linear regime. In particular, we consider the Gallavotti-Cohen theorem and the Evans-Searles theorem, which concern a class of molecular dynamics models, and we discuss the more recent theorems for stochastic evolutions, obtained by Jona-Lasinio and his collaborators.The predictions of these theorems have been tested numerically and experimentally.While illustrating the contents of the theorems, we analyze their connections and the hypotheses under which they are supposed to hold. We conclude pointing outseveral open problems.