This book provides an elementary introduction to probability theory and its applications. The emphasis is on essential probabilistic reasoning, amply motivated, explained and illustrated with a large number of carefully selected samples. The fourth edition adds material related to mathematical finance, as well as expansions on stable laws and martingales.

1: Set.- 2: Probability.- 3: Counting.- 4: Random Variables.- 5: Conditioning and Independence.- 6: Mean, Variance and Transforms.- 7: Poisson and Normal Distributions.- 8: From Random Walks to Markov Chains.- General References.- Answers to Problems.