In the early period of development of the chemistry of complexes, a number of theories were put forward in order to explain in a simple manner the formation of a new compound by the combination of two or more stable and apparently saturated compounds. Though these early theories were dot fully successful in many ways. yet they served their purpose of encouraging the workers in the field to perform new experiments in order to prove or disprove the theory. The slow accumulation of experimental data helped to understand the nature of chemical bond by the latter workers. However, the real credit goes to Werner who in 1893 introduced bold new ideas which explained a number of experimental facts. Though the modem theories are more encompassing in nature yet the basic postulates introduced y werner is still valid to this day. In the next four subsections we shall discuss four different approaches including Werner's to explain three main properties namely, geometry, and colour and the magnetic properties of the complexes.