The association between malignancy and thrombosis has long been appreciated but remains incompletely understood. This is the first of a two-part review of the complex, integral relationship between these two entities, and addresses the mechanisms and pathogenesis of this relationship and the clinical risk factors for thromboembolism in cancer patients. J. Surg. Oncol. 2011; 104:316–322. © 2011 Wiley-Liss, Inc.