Accurate interpretation of environments of sedimentation and of resulting sedimentary-rock bodies is recognized as fundamentally important in connection with exploration for oil and gas. For this there is need not only for identification of deposits favorable as source rocks but understanding of factors involved in making stratigraphic traps or affecting migration and accumulation of oil and gas. Analysis of data pertinent to paleoecologic studies may be divided into parts devoted to a review of several basic concepts concerned with stratigraphic variations and a survey of advances in paleoecologic methods observable in recent years. Extensive programs of research, especially of marine environments, now being conducted under auspices of the American Petroleum Institute an by several institutions in the Atlantic, Gulf of Mexico, and Pacific border regions and in Europe are valuable in adding firmer basis for interpretation of pre-Recent sedimentary deposits. A summary of this work and discussion of a few examples are presented.