Baikai is the deepest lake on earth and one of the most ancient. The pronounced endemism and specific wealth of its fauna andflora has attracted the keen interest of biologists and biogeographers all over the world. A start on the Baikai studies was made more than 200 years ago, but they have been carried on with the greatest intensity in the last 30 to 40 years, and more than 1,000 scientific works devoted to it have appeared in this period. Hence there is an urgent need for a summary of the main results of more than 200 years' study of one of the most remarkable lakes of our planet, and this the author has endeavoured to provide. A zoologist and hydrobiologist hirnself, he has concentrated on the living world of the lake. The author has for many years worked at Baikai as head of the Biologo-Geographical Institute and the Baikai Biological Station of Irkutsk University. In preparing this book for the press the author has received invaluable assistance from cartographer N.V. TYUMEN TSEV, algologists N.L. ANTIPOVAandO. M. KOZHOVA, hydrobiologists G. L. VASILYEVA, G. J. SHNYAGINA, L. J. PROTASOVA and R. A.