B cells and their products play a key role in the development and progression of cancers, with pro- and antitumoral functions in the tumor microenvironment and in circulation. This chapter summarizes the role of B cells in anti-tumor immune surveillance and malignant cell recognition, B cell origin and development, immunoglobulin gene rearrangement, isotypes and subclasses, elicitation of B cell immunity and B cell differentiation, cellular B cell immunity, and humoral immunity. Understanding B cell immunology is crucial to either promote or suppress B cell activity to favor tumor cell killing and explore novel therapeutic avenues for cancers.