Bicanonical and adjoint linear systems on surfaces of general type

This note contains a new proof of a theorem of Gang Xiao saying that the bicanonical map of a surface S of general type is generically finite if and only if the second plurigenus of S is strictly larger than 2. Such properties are also studied for adjoint linear systems |K_S+L|, where L is any divisor with at least 2 linearly independent sections.