During the past 20 years, the advent of neoadjuvant, primary, and adjuvant concurrent chemoradiotherapy has improved cancer care dramatically. Significant contributions have been made by technological improvements in radiotherapy, as well as by the introduction of novel chemotherapy agents and dosing schedules. This article will review the rationale for the use of concurrent chemoradiotherapy for treating malignancies. The molecular basis and mechanisms of action of combining classic cytotoxic agents (e.g. platinum-containing drugs, taxanes, etc.) and novel agents (e.g. tirapazamine, EGFR inhibitors and other targeted agents) with radiotherapy will be examined. This article is part one of two articles. In the subsequent article, the general principles outlined here will be applied to head and neck cancer, in which the impact of concurrent chemoradiotherapy is particularly evident.