<p>The use of probability theory and probabilistic process approaches is proposed when working with fuzzy sets, which are widely used in expert diagnostic systems and in controller design. A formula for calculating the probability of a fuzzy event is presented, based on the idea of identifying fuzzy events as fuzzy sets. A number of applications of this formula are considered by analogy with classical probability theory, namely, conditional, total, geometric probability, etc., as well as the use of Bayes and Bernoulli formulas. The approaches obtained are relevant for the construction of engineering systems and models in scientific and industrial fields.</p>