presents and evaluates Russell's arguments for two competing theories on the nature of particulars at different stages in his career: the substratum theory of particulars (1903-1913) and the bundle theory of particulars (1940-1948). Through its original focus on Russell's little known metaphysics in the later part of his career, this study explains why Russell's theory of particulars is relevant today. It argues that a Russellian realist bundle theory is indeed the best explanation of similarities and differences that we observe around us thanks to the ontological economy such a theory provides and its strength and completeness as a theory of the nature of reality. Tackling the major criticisms levelled against the realist bundle theory - the problem of individuation, the problem of necessity, and the problem of analyticity - this study presents and defends a tenable Russellian bundle theory which can answer the objections. is a novel and significant contribution to Russell scholarship.