Bessel and Mittag–Leffler functions are prominent within mathematical and scientific fields due to increasing interest in non-conventional models within applied mathematics. Since the analytical solutions of many differential and integral equations of arbitrary order can be written as series of special functions of fractional calculus, they are now unavoidable tools for handling various mathematical models of integer or fractional order. From Bessel to Multi-Index Mittag–Leffler Functions analyses this through the study of enumerable families of different classes of special functions.Enumerable families are considered and the convergence of series is investigated. Providing a unified approach to the classical power series, analogues of the classical results for the power series are obtained, and the conclusion is that each of the considered series has a similar convergence behaviour to a power series. Also studied are various properties of the Bessel and Mittag–Leffler functions and their generalizations, including estimations, asymptotic formulae, fractional differentiation and integration operators.