This paper reports on a study of 12 postgraduate research students' development of information search expertise. It focuses on students' perceived importance and growing understanding of different source types as they progress through their studies. Taking an in-depth longitudinal approach, this study makes use of surveys, direct observations of students' searches through various sources, students' verbalization of their thoughts while searching data-bases/sources and interviews during a one-year period. After analysing the data from the surveys and the interviews of the students with regard to their use of different source types, the paper identifies three stages of students' information needs, and links them to the progress of the students' research. The paper uncovers the roles that different source types play in different stages of research, and finds that while many students were initially unfamiliar with many source types, a growing understanding of them has contributed to the students' development of information search expertise. (Contains 6 notes and 5 tables.)