Given a transformation T of a standard measure space (X, µ), let M(T ) denote the set of spectral multiplicities of the Koopman operator U T defined in L 2 (X, µ)⊖C by U T f := f •T . It is discussed in this survey paper which subsets of N∪ {∞} are realizable as M(T ) for various T : ergodic, weakly mixing, mixing, Gaussian, Poisson, ergodic infinite measure preserving, etc. The corresponding constructions are considered in detail. Generalizations to actions of Abelian locally compact second countable groups are also discussed.The corresponding unitary isomorphism conjugates U T with the operator of multiplication by the independent variable z on T. 4 In fact, an extra condition on the realizable subsets was imposed in [KaLe]. However, as was shown later in [Da4], this condition is satisfied for all finite subsets of N. 5 Gaussian transformations are never of rank one [dR]. 6 In the infinite case, mixing does not imply ergodicity and ergodicity does not imply recurrence.