In this talk I survey the theoretical expectations for baryon number violation within the context of grand unified theories. I first give a classification of many of the possible baryon-number violating effective operators, including a discussion of typical processes that they induce, their selection rules, their relevant mass scales, and typical underlying mechanisms. I then specialize to the dimension-six four-fermion operators that describe the exchange of lepto-quark di-quark bosons in many popular models such as SU5, S010, E6, etc. After describing the general structure of these operators I discuss the estimate of the unification mass, the predictions for the nucleon lifetime, predictions for the branching ratios, and nuclear effects. Special emphasis is given to the related questions of which results are common to a wide class of theories and which types of measurements could discriminate between models.