In real-life situations one is almost never able to set up a range space and determine the sizes of the subsets so as to compute the probabilities needed for a probability distribution. This is true with discrete random variables and the situation becomes impossible if the random variable is continuous. How then is it possible to put the concepts discussed in the previous chapter to work? This question cannot be answered completely at this stage, but it can be said that at the heart of the procedure an assumption is made by the person responsible for the work that the real, but unknown, distribution is sufficiently similar to a theoretical distribution of known characteristics so that the theoretical distribution can be used as a substitute for the real one. The responsible person should have some idea of the nature of the real distribution and also a knowledge of the available theoretical distributions so that a rational matching can be made. The closer the match, the better the results. This chapter describes a number of discrete theoretical distributions. The next chapter is devoted to a number of continuous distributions. Neither is complete. However, with a knowledge of the distributions described one should be able to cope with virtually all problems in forest and natural resource inventory that require a probability distribution. For a more comprehensive coverage of probability distributions, see Johnson and Kotz (1969, 1970), Patel et al. (1976), and Hastings and Peacock (1974).