5-Fluorouracil (5-FU) is still the mainstay of chemotherapy in patients with metastatic colorectal cancer. A prolonged infusion of 5-FU is more active than any other schedule of 5-FU used to date. Cisplatin does not improve treatment results compared with 5-FU alone and is not recommended outside clinical trials. Biomodulation of 5-FU is a major step forward in the treatment of colorectal cancer patients and as the standard chemotherapy for advanced colorectal cancer. Two schedules of folinic acid daily for 5-day (low and high doses) and weekly high dose in combination with daily or weekly 5-FU are the most widely used schedules. Although the response rates to either schedule are comparable, the profile of toxicity is different, being stomatitis for the daily schedule and diarrhea for the weekly schedule as the dose-limiting toxicity. Modulation of 5-FU by methotrexate is time dependent. An interval of 24 hours between methotrexate and 5-FU is necessary for effective modulation. Other modulators, like interferon and N-phosphonoactyl-L-aspartate (PALA), are promising treatment options currently under investigation in randomized trials. The data from phase II and III trials using modulation of 5-FU by folinic acid, PALA, or methotrexate, or using continuous infusion 5-FU indicate that all of these strategies are active. Randomized trials are currently underway to further investigate these therapeutic approaches and whether a specific modulation offers more therapeutic advantages.