Given an ergodic flow (Tt) t∈R we study the problem of its self-similarities, that is, we want to describe the set of s ∈ R for which the original flow is isomorphic to the flow (Tst) t∈R . The problem is examined in some classes of special flows over irrational rotations and over interval exchange transformations. In particular, translation flows on translation surfaces are considered: we prove that under the weak mixing condition the set of self-similarities has Lebesgue measure zero. For von Neumann special flows over irrational rotations given by Diophantine numbers, this set is shown to be equal to {1}, while for horocycle flows a weak convergence in case of some singular (with respect to the volume measure) measures is shown to give rise to some new equidistribution result. The problem of self-similarity is also studied from the spectral point of view, especially in the class of Gaussian systems.Contents SI(T ) = {s ∈ R * : T and T s are spectrally isomorphic}.Clearly, SI(T ) is a multiplicative subgroup of R * , it contains I(T ) and since T preserves the space of real-valued functions, also −1 ∈ SI(T ). If T is spectrally self-similar, that is, SI(T ) = {−1, 1} and SI(T ) has positive Lebesgue measure, then T has pure Lebesgue spectrum (see Proposition 8.6). On the other side, if T has singular continuous spectrum, then SI(T ) has zero Lebesgue measure and T s is spectrally disjoint from T for almost all s (Theorem 8.4). Moreover, T s and T t are spectrally disjoint for almost all (s, t) ∈ R 2 . The spectral disjointness results were inspired by a discussion of the second author with B. Host in 1994. † In Section 9, we construct ergodic flows that are not self-similar in the unitary category (see Theorem 9.4 and Remark 9.5). For this purpose, we deal with Gaussian systems, that is dynamical systems that are completely determined by the spectral measure of the underlying Gaussian process. A construction of a measure that is supported by a set, which emulates a classical Kronecker set yields a Gaussian flow T with simple spectrum such that SI(T ) = {−1, 1} and T s is spectrally disjoint from T for s = ±1. Moreover, for some countable multiplicative symmetric subgroups G ⊂ R * a modification of the construction yields † B. Host showed Lemma 8.2 below for continuous singular measures on R.