This paper examines research on students' learning in small tutorial groups, highlighting: (1) cognitive processes elicited by small group discussion and their effects on achievement; and (2) the influence of tutors on student learning. The studies examined problem based learning (PBL) in small groups in high schools and universities. Researchers investigated whether: PBL activated students' prior knowledge; initial analysis of problems stimulated retrieval of knowledge acquired earlier; and prior knowledge facilitated understanding of new information. They noted the effect of prior knowledge activation on processing new information; contributions of group discussion to the effect of PBL; cognitive processing while involved in problem discussion; and evidence for constructive processes in small group tutorials. They also investigated the extent to which tutors should be subject matter experts, noting behaviors that characterized effective tutors. They examined research on: expert and non-expert staff tutors; staff and student tutors; reasons for inconclusive results of tutor expertise studies; differences in actual behavior between tutors; and theories of effective tutors. Results suggest that initial analysis of a problem mobilizes students' prior knowledge that is used to construct an initial representation of the processes responsible for the phenomena or events described in it. Students' construction of this initial theory facilitates the comprehension of problem-relevant new information. Social congruence, subject matter expertise, and the ability to be cognitively congruent with one's students are all crucial to effective tutoring. (Contains 30 references.) (SM)