These lecture notes, edited by A. Dold and B. Eckmann, present a comprehensive treatment of Orlicz spaces and modular spaces, authored by Julian Musielak. The book is structured into five chapters, covering modular spaces, Orlicz spaces, countably modulated spaces, families of modulars depending on a parameter, and applications of modular spaces. The first version of these notes was published in Polish in 1978, and the current text expands on this with the theory of generalized Orlicz spaces. The focus is on scalar-valued function spaces, with an emphasis on modular convergence rather than norm convergence. The text includes detailed sections on conjugate modulars, F-modular spaces, bimodular spaces, modular bases, embeddings of generalized Orlicz classes, compactness in spaces $ E^p $, generalized Orlicz-Sobolev spaces, uniform convexity of $ L^p $ spaces, disjointly additive modulars, complementary functions, interpolation of operators, countably modulated spaces, spaces of infinitely differentiable functions, $ \varphi $-integrable functions, families of modulars depending on a parameter, and applications to integral equations and approximation theory. The book also includes comments, a bibliography, and an index. The work is published by Springer-Verlag in 1983, and it is protected by copyright. The author acknowledges the contributions of various individuals who helped in the preparation of the text. The book is intended for readers interested in generalized Orlicz spaces, with a focus on modular convergence and its applications in functional analysis.These lecture notes, edited by A. Dold and B. Eckmann, present a comprehensive treatment of Orlicz spaces and modular spaces, authored by Julian Musielak. The book is structured into five chapters, covering modular spaces, Orlicz spaces, countably modulated spaces, families of modulars depending on a parameter, and applications of modular spaces. The first version of these notes was published in Polish in 1978, and the current text expands on this with the theory of generalized Orlicz spaces. The focus is on scalar-valued function spaces, with an emphasis on modular convergence rather than norm convergence. The text includes detailed sections on conjugate modulars, F-modular spaces, bimodular spaces, modular bases, embeddings of generalized Orlicz classes, compactness in spaces $ E^p $, generalized Orlicz-Sobolev spaces, uniform convexity of $ L^p $ spaces, disjointly additive modulars, complementary functions, interpolation of operators, countably modulated spaces, spaces of infinitely differentiable functions, $ \varphi $-integrable functions, families of modulars depending on a parameter, and applications to integral equations and approximation theory. The book also includes comments, a bibliography, and an index. The work is published by Springer-Verlag in 1983, and it is protected by copyright. The author acknowledges the contributions of various individuals who helped in the preparation of the text. The book is intended for readers interested in generalized Orlicz spaces, with a focus on modular convergence and its applications in functional analysis.