This book is an introduction of Southeast Asian Desmids, and deals with about 950 taxa of 34 genera which includes many taxa observed previously by the present authors and other coauthors, some species described as new to science from Southeast Asia. The book gives details of morphology of various Desmids cell and terminology for the characteristic parts of the cell are given with figure of Cosmarium cell. Then, cell shapes differing in genus or in each alga are shown with figure and comparative table. Various shapes of semi cell, cell and extra margin, ornamentation of cell wall are also given in figures. The cell shapes of Desmids are found having much diversity. The chloroplasts are shown in Figures. Asexual reproduction and sexual reproduction of Desmids are very unique compared with other algal groups. The processes of reproduction are explained rightly with figures. Later habitat of Desmids, collection and preservation of Desmids samples, observation by microscope, record of observation, explanations to scientific name of Desmids used in this book are given. Furthermore, Key to Genera & Systematic Enumeration and explanation for each species and figures are described continuously, providing other details of location & distribution.