Biology is a science on life. This definition, concise and most commonly used, is satisfactory for almost everybody. It is otherwise when one asks: What is life? Then it appears that no one feature can be indicated which distinguishes “the living” from “the non-living.” The author presents the sources of these difficulties and then gives his own attempt to solve the problem of definition of live—which is based on the idea of levels of the biological organization. In author’s view, to characterise the objects of research in biology we should apply not one concept of life (or of living organism) but three concepts: of organized biological matter (for the molecular and sub-cellular levels), of living organism (for the level of the specimen), and of life (for the sphere of phenomena which occur on the population-species-biocenotic level).