Non-cooperative games (or strategic games) are mainly studied through two models: normal form or extensive form. The latter will be presented in Chapter II. The former describes the choice spaces of each player and the result of their common choices. This is evaluated in terms of the players' von Neumann–Morgenstern utilities (i.e., the utility of a random variable is its expected utility (von Neumann and Morgenstern, 1944, Chapter I, 3.5)), and hence the following definition:

A normal form game is defined by a set of I, strategy spaces Si, i ∈ I, and real payoff functionsFi, i ∈ I, on S = Πi Si.

It is finite (or a bi-matrix game if #I = 2) if I and all Si are finite.

Under suitable measurability conditions one defines the mixed extension of a game G = (I, (Si, Fi)i∈I) as the game Γ = (I, (Σi, φi)i∈I), where Σi is the set of probabilities on Si and φi (σ) = ∫SFi(S) Πi∈I σi(dsi). An element of Si will be called a pure strategy, while an element of Σi will be called a mixed strategy (of player i). Unless explicitly specified (or self-evident), the following definitions are always used on the mixed extension of the game.

An (ε-)equilibrium is an I-tuple s such that for every i, si is an (ε-)best reply to s−i.