The paper deals with the development of the electrolytic condenser which is now so widely used in modern radio receivers and in amplifying apparatus.A brief historical survey is given to indicate the lines of development which have led up to the modern designs. This is followed by descriptions of the construction and chief electrical properties of the various types of condensers which are now used. These properties influence to a very great extent the manner in which the condenser can most economically and usefully be employed.Various theories have been put forward to explain the action of electrolytic condensers, but some of these do not how appear to be adequate. A critical summary is given, with an expression of views as to the most likely explanation.The manufacturing methods are treated at some length, together with the considerations which influence the employment of these condensers in various practical applications. A standardized method of test is recommended to secure uniformity in assessing their properties.