In this book the reader is provided with an overview of the most interesting ideas in Competitive General Equilibrium Theory as well as Welfare Economy. In doing so, the author adopts a methodological approach of a closed and inter-related system where all variables are simultaneously determined. By using theorems of fixed point, revealed preference and local and global stability, the author proves the existence of unicity and stability. Furthermore, he analyses those situations where the competitive system does not allocate resources efficiently. In this sense, he examines allocation problems in regard to property rights, information problems and choice and negotiation problems. Finally, the text includes a chapter that presents a topical guide of questions related to Social Choice.