This chapter relaxes the assumption of determistic population growth. It starts with a motivation for the inclusion of stochastic elements in population models and introduces two types of stochasticity: environmental stochasticity and demographic stochasticity. After this, three main probability distributions are introduced: the normal, the binomial and the Poisson. Using these distributions, the dynamics of a small population are simulated and the simulation outcome analysed statistically to assess the viability of the population. A fourth section provides some mathematical background of stochastic population models and population viability analysis. The chapter concludes with an outline of the International Union for Conservation of Nature's risk model to assess the endangerment of species.