Today, low temperature membrane reactors are used at various levels of development in biotechnology and environmental protection, the chemical and petrochemical industry, energy production, etc. The aim of this review is, first at all, to provide the reader with a clear understanding of the unified approach that can be followed in the design of a membrane reactor and of the various aspects that can unite both high and low temperature membrane reactors, taking into account the specificity of the case proposed. Afterwards the role of the membrane in a membrane reactor and the various functions that a membrane reactor can cover for the development of a process are described, mentioning also the most recent membrane bioreactors and photo catalytic membrane reactors, which will be included in the more general concept of a membrane reactor. Finally, an overview of the main uses to which low temperature membrane reactors can be applied is also provided.