Adjuvant chemotherapy for microscopic disease following eradication of clinically detectable lesions by primary surgery and/or radiotherapy is of documented benefit for some oncology patients. However, for many primary cancers efficacy is limited to specific subgroups of patients or has demonstrated no advantage over primary therapy alone. The rationale for adjuvant chemotherapy and results of selected trials are reviewed. In patients for whom adjuvant therapy is of demonstrated benefit, further trials aimed at delineation of patient selection factors, optimal chemotherapy regimens and schedules, and duration of therapy are needed; progress in adjuvant treatment of other subgroups may require development of more effective antineoplastic drugs, in addition to exploration of these factors.