Probability is the branch of mathematics that deals with events whose individual outcomes are unpredictable, but whose outcomes on average are predictable. In this chapter we shall describe the rules of probability. We shall apply these rules to specific situations. As you will see, the notions and methods of calculus play an extremely important part in these applications. In particular, the logarithmic and exponential functions are ubiquitous. For these reasons, this chapter has been included in this book.