The contribution sketches several ways of considering systems from the position of their modularity through viewing systems without any attention focused to their modularization, then as composed from functionally specified modules, up to the post-modular systems consisting of relatively independent autonomous modules sharing a common environment and acting in it. A relatively simple, uniform and productive theoretical framework for study of the mentioned aspects of systems behavior and modularity - the framework of the theory of grammar systems - will be presented, illustrated and discussed in certain details.