Abstract. The problem we deal with in this paper is the research of upper and lower bounds on the randomness required by the dealer to set up a secret sharing scheme. We give both lower and upper bounds for infinite classes of access structures. Lower bounds are obtained using entropy arguments. Upper bounds derive from a decomposition construction based on combinatorial designs (in particular, t-(v, k, )~) designs). We prove a general result on the randomness needed to construct a scheme for the cycle C,~; when n is odd our bound is tight. We study the access structures on at most four participants and the connected graphs on five vertices, obtaining exact values for the randomness for all them. Also, we analyze the number of random bits required to construct anonymous threshold schemes, giving upper bounds. (Informally, anonymous threshold schemes are schemes in which the secret can be reconstructed without knowledge of which participants hold which shares.)