Ernst Mayr, for whom, species and species problem have been the main concern of research, points out that speciation, the multiplication of species, that is division of one parent species into several daughter species is one process responsible for the evolutionary diversity of organic world. Darwin no where in his book Origin of Species defined the word species precisely although he interpreted speciation in terrns of reproductive isolation (Darwin again did not use the term reproductive isolation in his book.) Darwin did en~phasise that occupation of an unique ecological niche by each species could be a major characteristic of speciation. In this unit we shall briefly analyse the species concept laying emphasis on biological species and discuss in detail different types of mechanisms of speciation. Further we shall briefly look into how isolation brings about genetic repatterning and proceed to discuss the mechanisms of isolation with well chosen examples. As an illustration of how isolating mechanisms develop gradually, we shall explain the concept of ring species - the evolution of distinct new species. Finally there will also be a discussion on a phenomenon called genetic drift which however has no causal connection with speciation process but explains that in peripheral isolates gene frequencies drift due smapling error.