Follicular lymphoma (FL) represents the most common low-grade Non-Hodgkin''s lymphoma. FLs are of B cell origin, generally incurable, and frequently transform to high-grade diffuse large B cell lymphoma (DLBCL). Transformation is generally correlated with poor prognosis and the molecular mechanisms involved are largely unknown. This book gives an introduction to the field of carcinogenesis with focus on the events leading to Non-Hodgkin''s lymphoma and presents the results of a study aimed at identifying novel genes differentially expressed in patient-matched biopsies of FL and DLBCL to increase the understanding of disease progression. A general discussion of the methods applied, including the subtraction methodology cDNA Representational Difference Analysis (cDNA RDA), and the findings from this study are included. This book should be of interest to anyone who wants to get an understanding of the processes causing normal cells to transform into cancer cells and how studies of gene expression can highlight these issues.