Over the past decades a variety of different approaches to realize Compressed Air Energy Storage (CAES) have been undertaken. This article gives an overview of present and past approaches by classifying and comparing CAES processes. This classification and comparison is substantiated by a broad historical background on how CAES has evolved over time from its very beginning until its most recent advancements. A broad review on the variety of CAES concepts and compressed air storage (CAS) options is given, evaluating their individual strengths and weaknesses. The concept of exergy is applied to CAES in order to enhance the fundamental understanding of CAES. Furthermore, the importance of accurate fluid property data for the calculation and design of CAES processes is discussed. In a final outlook upcoming R&D challenges are addressed. (C) 2016 Elsevier Ltd. All rights reserved.