Inflammation is thought to be one of the major contributors to carcinogenesis. Accumulated studies in this field revealed that free radicals produced by inflammatory cells not only cause direct damage to DNA but also exert indirect effects such as de-regulation of cell proliferation and apoptosis, stimulation of angiogenesis, and modification of gene/protein expressions and protein activities, all of which are a critical step toward carcinogenesis. Free radicals have also been reported to act as both initiator and promoter of carcinogenic process. Recent evidence shows that free radicals convert benign tumors to more malignant ones (i.e. tumor progression) leading to the final stage of carcinogenesis. This article reviews the current findings linking inflammation and cancer, and shed light on inflammatory cell-derived free radicals as major endogenous reactive substances for tumor development and progression.