Given a graph G and a subset W ⊆ V (G), a Steiner W -tree is a tree of minimum order that contains all of W. Let S(W ) denote the set of all vertices in G that lie on some Steiner W-tree; we call S(W ) the Steiner interval of W. If S(W ) = V (G), then we call W a Steiner set of G. The minimum order of a Steiner set of G is called the Steiner number of G.Giventhe set of all vertices in G lying on some u-v geodesic, and let J [u, v] denote the set of all vertices in G lying on some induced u-v path. Given a set S ⊆ V (G), let I [S] = u,v∈S I [u, v], and let J [S] = u,v∈S J [u, v]. We call I [S] the geodetic closure of S and J [S] the monophonic closure of S. If I [S] = V (G), then S is called a geodetic set of G. If J [S] = V (G), then S is called a monophonic set of G. The minimum order of a geodetic set in G is named the geodetic number of G.In this paper, we explore the relationships both between Steiner sets and geodetic sets and between Steiner sets and monophonic sets. 140 C. Hernando et al. / Discrete Mathematics 293 (2005) 139 -154 and the geodetic number, and address the following questions: in a graph G when must every Steiner set also be geodetic and when must every Steiner set also be monophonic. In particular, amongothers we show that every Steiner set in a connected graph G must also be monophonic, and that every Steiner set in a connected interval graph H must be geodetic.