  This is a short review of the Kadomtsev-Petviashvili hierarchies of types B
and C. The main objects are the $L$-operator, the wave operator, the auxiliary
linear problems for the wave function, the bilinear identity for the wave
function and the tau-function. All of them are discussed in the paper. The
connections with the usual Kadomtsev-Petviashvili hierarchy (of the type A) are
clarified. Examples of soliton solutions and the dispersionless limit of the
hierarchies are also considered.
