Irinotecan, a water-soluble, semisynthetic derivative of camptothecin, is a key component of first- and second-line treatment regimens for metastatic colorectal cancer (CRC). In the first-line treatment of metastatic CRC, the results of two prospective, multicenter phase III trials have shown that the combination of irinotecan with bolus or infusional 5-fluorouracil (5FU)/leucovorin (LV) can significantly prolong survival compared with 5FU/LV alone, with a manageable side effects profile. In addition, irinotecan-based regimens, with or without oxaliplatin, may improve resectability of metastases and further increase patient survival. Studies of irinotecan in the first-line setting in combination with newer agents, such as bevacizumab, have shown impressive overall survival. In the second-line setting, irinotecan has demonstrated efficacy superior to that of best supportive care. Initial studies of irinotecan plus bolus 5FU/LV, and the preliminary results from trials of irinotecan plus infusional 5FU/LV in the adjuvant setting, have been disappointing; however, for the largest trial, the Pan-European Trial in Adjuvant Colon Cancer, results with sufficient follow-up are pending. Irinotecan has an acceptable tolerability profile and is not associated with cumulative toxicities in patients with metastatic CRC; regimens containing irinotecan extend treatment duration and improve survival. New regimens and adjunctive therapies are being explored to reduce the incidence of common complications of irinotecan treatment, such as diarrhea and neutropenia.