Kadomtsev–Petviashvili hierarchies of types B and C

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. Connections with the usual (type-A) Kadomtsev–Petviashvili hierarchy are clarified. Examples of soliton solutions and the dispersionless limit of the hierarchies are also considered.