This paper describes research on dialogue between teachers and pupils during primary school science lessons, using talk from two classrooms to provide our examples. We consider whether teachers use dialogue to make education a cumulative, continuing process for guiding the development of children's understanding. Case studies of two teachers, using observational data taken from a larger data set, are used to illustrate their use of talk as a pedagogic tool. We also consider the differing extent to which the two teachers highlight for pupils the educational value of talk, and the extent to which they attempt to guide pupils' own effective use of talk for learning. Implications are drawn for evaluating the ways teachers use dialogue, and for professional development. An example is provided of an activity which has been found to help teachers implement dialogic teaching, and which illustrates how such an approach involves organising the structural variety of talk.