This book is concerned with the analysis of count data, in particular with cross-classified categorical data using log-linear models and with logistic regression. Fienberg and Rinaldo (Journal of Statistical Planning and Inference, 137, 3430–3445 (2007)) explore the last three centuries of work on the subject. In this chapter we review basic information on conditional independence, random variables, expected values, variances, standard deviations, covariances, and correlations. We also review the distributions most commonly used in the analysis of contingency tables: the binomial, the multinomial, product multinomials, and the Poisson. Christensen (Analysis of variance, design, and regression: Applied statistical methods. Chapman and Hall. 1996, Chap. 1) contains a more extensive review of that material not being introduced specifically for dealing with count data.