  A subset $S$ of initially infected vertices of a graph $G$ is called forcing
if we can infect the entire graph by iteratively applying the following
process. At each step, any infected vertex which has a unique uninfected
neighbour, infects this neighbour. The forcing number of $G$ is the minimum
cardinality of a forcing set in $G$. In the present paper, we study the forcing
number of various classes of graphs, including graphs of large girth, $H$-free
graphs for a fixed bipartite graph $H$, random and pseudorandom graphs.
