The book examines the basic concepts and theorems of probability theory using simple examples. It is based on a combinatorial approach, but along with the classical definition of probability, a statistical definition is also introduced. The model of random walk on a line, which describes the physical process of one-dimensional Brownian motion of particles, as well as other examples, is analyzed in detail. Simple statistical problems are discussed. For students and teachers of secondary schools, lyceums and gymnasiums, for leaders and participants of mathematical circles, as well as for everyone who is interested in mathematics.