In about one decade's time the number of apartments built in the world by a new system developed in Yugoslavia has exceeded 20,000. This patented system, known as the Zezelj system, is a means of producing the smallest possible number of components parts capable of being assembled into the greatest possible variety of structures. It gives the architect ample freedom to design a structure as he chooses and give the building as a whole any desired architectural treatment. Their behavior is that of an ordinary reinforced concrete skeleton system, cast in place, but the use of post-tensioning allows either jobsite or industrial production of precast components, with simple and quick assembly and high rationality of construction. For that reason the writer believes that this system is a most suitable solution for the world's housing shortage. The basic concept is one of uniting all precast assembly components, columns and ceiling slabs into one monolithic whole by post-tensioning. The post-tensioning tendons run along the contiguous slabs and through holes prepared for this purpose beforehand in the columns. Post-tensioning is done in both the longitudinal and transverse directions. The tendons can either be straight or have the necessary eccentricity depending on the grid of the ceiling slabs and the design live load. For residential structures a 14 by 14 grid is most suitable, but all combinations up to 23 by 23 feet are possible. Horizontal loads are taken by vertical concrete diaphragms provided between the columns in the necessary number and extending continuously from the foundations to the top of the building.