In both children and adults, the natural history of Crohn's disease (CD) is characterized by relapsing and remitting bouts of intestinal inflammation, often associated with a progressive shift from inflammatory to complicated stricturing or penetrating disease behavior. The past 2 decades have seen a dramatic shift in therapeutic approach with the increasingly common use of early thiopurine immunomodulation. These maintenance medications were initially introduced primarily as corticosteroid-sparing agents capable of minimizing recurrent flares of inflammatory disease and have proven to be quite efficacious. Increasing evidence suggests, however, that thiopurines may only delay rather than prevent the development of complicated disease behavior. Data from both adult and pediatric CD populations from around the world are reviewed in terms of the effect of early immunomodulation on progression to complicated disease behavior, need for surgery, and prevention of recurrent disease after resection. The effect of thiopurines on the growth of children is also reviewed.