This is the first book to systematically study the weak systems of mereology. In its chapters, the author critically analyzes and explains core topics related to mereology, such as parthood without antisymmetry, non-existence of the zero element, and Leśniewski's notion of class and set. The book also delves into three theories of parthood: two concern the sum existence axioms, and the third contends with transitivity of parthood. This is the first systematic analysis of systems of mereology of its kind and is suitable for students, scholars, logicians, and mathematicians who wish to further their knowledge of mereology. Original polish publication "Podstawy teorii części" by The Nicolaus Copernicus University Press.