In class previous classes we have studied about the probability as a measure of uncertainity of various phenomenon in our daily life. Normally we obtain the probability of an events as the ratio of the number of favourable outcomes to that of the total possible outcomes. This is called classical theory of probability. Up to class IXth we have studied statistical theory of probability. But the drawback of these theory is that it cannot be applied to the activities which have infinite numbers of outcomes. In this chapter we will study about this approach which is called axiomatic approach of probability. To understand this we have to understand the basic concept.