The concept of capacity — plant capacity, industrial capacity or national economic capacity — frequently turns up in economic discussion. It is often referred to as though people generally know its meaning, yet it is an illusive and ill-defined concept. The history of our subject contains sporadic bursts of interest in the matter of capacity, but the present state of treatment of this concept is in need of further development. The present essay attempts to stimulate the debate once again and to set out some new aspects of statistical treatment.