Probability can be defined as a measure of uncertainty of various events. This can be defined using two major types of theories namely classical theory of probability and statistical approach of probability. In simple terms, the probability is the ratio of the number of outcomes favourable to the event, to the total number of outcomes. The related terms viz. experiment, sample space, events, outcomes and so on. Algebra of events explains the intersection, union, difference and complement of events, etc. The chapter discusses the concept of probability as a measure of uncertainty of various phenomena or simply, chances of occurrence of an event. In this chapter, students will learn the concepts such as random experiments; outcomes, sample spaces (set representation) and Events; occurrence of events, ‘not’, ‘and’ and ‘or’ events, exhaustive events, mutually exclusive events, Axiomatic (set theoretic) probability, connections with other theories studied in earlier classes. Probability of an event, the probability of ‘not’, ‘and’ and ‘or’ events are also covered in the chapter.