Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization. When visible to the naked eye, it can be a small test (shell) of a microfossil (radiolarian or foraminifer), a phosphatized shark tooth, basalt debris or even fragments of earlier nodules. They generally range in size from 0.5 cm to 25 cm in diameter but the average diameter being 2 to 4 cm. However, the largest nodule ever recorded weighed about 800 kg. The shape of nodules vary from spheroidal to oblate, discoidal or prolate. Mostly they are asymmetrical. Truly symmetrical shapes are quite rare. Near spherical shapes have been observed in smaller nodules. It may be said that the shape of the nodules is determined by their growth pattern and is a reflection of gross internal structures and the proximity of the nodules to the mud water interface. The morphology of the nodules depend to a large extent upon the shape of nucleus and is often responsible for the enrichment of the metals. In the present paper, an attempt has been made to classify the most common type of nodules found in the Central Indian Ocean Basin alongwith their genesis and chemistry.