Introduction - The theory of legal system is primarily the study of four problems (1) The problem of existence - what are the criteria for the existence of a legal system? (2) The problems of identity and membership - what are the criteria determining which laws belong be a given system? (3) The problem of structure - Is there of necessity a common internal structure to all legal systems? An internal structure is a structure at internal relations. An internal relation exists between laws if and only if one at them affects the existence or meaning of the other. (4) The problem of content - is there of necessity a common content to all legal systems? This thesis is an introduction to the study of the first three problems. It aims to give them precise formulation, demonstrate their importance, examine some views concerning their solution which were advanced in the past and make a start on another attempt to solve them.