The great progress which has been made in recent years in the field of the chemical and physical properties of water and the dependence of the life processes on these makes it appear desirable to take water as an environment as the central theme and this subject occupies half the text. This book has grown out of the course in Hydrobiology which has been given at the Biological Station at Lunz for some decades.This circumstance will make it clear why the text may seem to be overburdened with examples drawn from conditions in the lakes of Lunz and from work carried on at the Lunz Station.