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. A straight line random walk model describing the physical process of one-dimensional Brownian particle motion is analyzed in detail, along with other examples. Simple statistical problems are discussed. For students and teachers of secondary schools, lyceums and gymnasiums, for leaders and participants in mathematical circles, as well as for everyone who is interested in mathematics.