This paper on adult informal learning is divided into four sections. Section 1 examines different conceptions of informal learning and the issues and limitations associated with alternative definitions of informal learning. Section 2 is a review of empirical research on the estimated extent, role, and outcomes of informal learning and posited linkages between informal and formal methods of learning. It reports that, according to the New Approaches to Lifelong Learning (NALL) 2000 national survey, over 95 percent of Canadian adults are involved in some form of informal learning activities that they identify as significant. Section 3 critically assesses current research approaches to studying informal learning and identifies policy-relevant knowledge gaps concerning the general level and nature of informal learning, distribution of informal learning across the adult population, impact of informal learning on individual and firm performance, and relationship of informal learning to formal skills development. Section 4 recommends optimal approaches to future research on informal learning practices with a particular focus on survey research in Canada and finds it imperative to establish benchmarks of the general incidence, basic contents and modes, and any differential patterns of intentional informal learning and training, and to continue to track trends in relation to other dimensions of adult learning. (Appendixes include NALL questions and 69 references.) (YLB)