  Proton decay is usually discussed in the context of grand unified theories.
However, as is well-known, in the standard model effective theory proton decay
appears in the form of higher dimensional non-renormalizable operators. Here,
we study systematically the 1-loop decomposition of the $d=6$ $B+L$ violating
operators. We exhaustively list the possible 1-loop ultra-violet completions of
these operators and discuss that, in general, two distinct classes of models
appear. Models in the first class need an additional symmetry in order to avoid
tree-level proton decay. These models necessarily contain a neutral particle,
which could act as a dark matter candidate. For models in the second class the
loop contribution dominates automatically over the tree-level proton decay,
without the need for additional symmetries. We also discuss possible
phenomenology of two example models, one from each class, and their possible
connections to neutrino masses, LHC searches and dark matter.
