  We analyze the bootstrap percolation process on the stochastic block model
(SBM), a natural extension of the Erd\"{o}s--R\'{e}nyi random graph that allows
representing the "community structure" observed in many real systems. In the
SBM, nodes are partitioned into subsets, which represent different communities,
and pairs of nodes are independently connected with a probability that depends
on the communities they belong to. Under mild assumptions on system parameters,
we prove the existence of a sharp phase transition for the final number of
active nodes and characterize sub-critical and super-critical regimes in terms
of the number of initially active nodes, which are selected uniformly at random
in each community.
