This paper reviews recent research on the characterization, properties, and definition of dietary fiber as well as its possible role in colonic carcinogenesis and diverticulosis. Despite progress in analytic methods and characterization, an accepted definition and terminology for fiber are lacking as is an accurate, rapid method for measurement of total dietary fiber or fiber in foods. Mechanisms of effects of fiber in the gut and the significant of interactions between fiber, nutrients gut flora and associated metabolites, and enteric secretions are unclear. Epidemiologic and experimental data indicate an increased risk of diverticular disease and colonic cancer with low-fiber intakes; however, genetic, environmental, cultural, dietary, and other variables were often uncontrolled in the epidemiologic studies. Thus, conclusive evidence for a causal relationship between low intake of fiber and diverticulosis or colonic cancer is not available, and the question whether first protects against human colonic cancer and/or diverticulosis is not completely resolved. Clinical trials in which symptomatic diverticular disease was treated with supplementary dietary fiber have generally had favorable results. Numerous specific questions require additional study before a role for dietary fiber in the prevention of human colonic diverticulosis and cancer of the colon can be established. Suggestions for possible future investigation are provided.