In recent years, we have made huge strides in our understanding of the molecular complexity of diffuse large B-cell lymphoma (DLBCL). New technologies, such as gene expression profiling, RNA interference screening, and DNA sequencing, have identified several new signaling pathways and therapeutic targets for drug development. While we once considered DLBCL to be a single disease entity, recent insights have helped identify the existence of at least three distinct molecular diseases: a germinal center B-cell-like subtype, an activated B-cell-like subtype, and a primary mediastinal B-cell lymphoma subtype. All three subtypes originate from different stages of B-cell differentiation and are characterized by distinct mechanisms of oncogenic activation. This classification of DLBCL has laid the foundation for the development of new agents and novel strategies that target individual subtypes.