During the 19th, and for most of the 20th century, malignant tumors were removed by mutilating radical anatomic dissection. Advances such as anesthesia, asepsis, and blood transfusion made possible increasingly more radical operations. There was no scientific rationale for the operations being performed. Surgery in the 20th century was dominated by the principles of William S. Halsted, who contended that the bloodstream was of little significance as a route of tumor cell dissemination; a tumor was autonomous of its host; and cancer was a local-regional disease that spread in an orderly fashion based on mechanical considerations. Halsted believed that both the extent and nuances of an operation influenced patient outcome and that inadequate surgical skill was responsible for the failure to cure. A new surgical era arose in 1957, when cancer surgery began to be influenced by laboratory and clinical research, with results contrary to Halstedian principles. A new hypothesis resulted in a scientific basis for cancer surgery. Clinical trials supported the thesis that operable cancer is a systemic disease and that variations in local-regional therapy are unlikely to substantially affect survival. Complex host-tumor relationships were shown to affect every aspect of cancer and, contrary to Halsted's thesis, the bloodstream is of considerable importance in tumor dissemination. Clinical trials also have shown that less radical surgery is justified. Studies have shown that improved survival can be achieved with systemic therapy after surgery. Such therapy can reduce both the incidence of distant disease and the tumor recurrence at the tumor site after minimal surgery. The use of systemic therapy in patients who have no identifiable metastatic disease is a drastic departure from previous strategies. New technological innovations resulting from engineering research have improved the quality of life of patients by eliminating the need for some surgical procedures. Because cancer is apt to be a systemic disease, however, clinical trials are necessary to determine the effect of these modalities on patient outcome. Although technological developments will continue to play a role in cancer therapy, research in molecular biology and genetics will dictate the future status of cancer treatment and, ultimately, the future of surgery.