The concept of Paramāṇu (atom) has been the fundamental of Philosophy and Science. All the Eastern and Western thinkers have thought a lot of the basic elementary particles of the physical world. Consequently different theories have originated and flourished. Some of them have been indicated in the present chapter. In the field of science, many astonishing researches and experiments on material world have been made. The first three decades of our century changed the whole situation in Physics radically. Two separate developments—that of relativity theory and Quantum physics - shattered all the principal concepts of the Newtonian world view: the notion of absolute space and time, the elementary solid particles, the strictly causal nature of physical phenomena, and the idea of an objective description of nature. None of these conceptions could be extended to the new domains into which Physics is now penetrating. So far as the philosophical worldview is concerned, it keeps some different as well as identical conceptions with those of Modern Physics. The identity and difference have been shown on the basis of the Bh.S here.