Digital image collections are expanding day by day, and image retrieval becomes even harder. Both individuals and institutions encounter serious problems when building their image archives and later when retrieving the archived images. Visual information cannot be fully expressed in words and normally depends on intuitive human perception. Consequently, this causes us to find the plain text-based information inadequate, and as a result, increases the value of the visual content. However describing, storing and retrieving the visual content is not simple. The research activities in this area, which escalated in the 90’s, have brought several solutions to the understanding, design and development of the image retrieval systems. This article reviews the studies on image retrieval systems in general, and content-based image retrieval systems specifically. The article also examines the features of content-based image retrieval systems.