Sunlight is part of everyday life and we accept it as good-and good it is in a number of ways. The sun is our source of warmth, and of the light by which we see. It is, in fact, the source of the energy with which life continues on earth. It furnishes energy for photosynthesis, and the pro­ ducts of photosynthesis constitute our food, building materials, and fuel. A steady state of balance and fine interrelationships exists between life on earth and all the forces and stresses in nature. This book will pin­ point the balance and relationships we share with sunlight. Our primary focus will be on the ultraviolet radiation of the sun, and on the ultraviolet photobiology of life on earth. This is the story of the effects of the sun's ultraviolet radiation, both good and bad, on all of us and all of life. We will explore the nature of the sun's ultraviolet radiation as it reaches the earth's surface today, and as it probably affected the earth in the distant past; and examine the effect of such radiation on all life, unicellular organisms as well as multicellular plants and animals. The effects of the sun's ultraviolet rays are primarily a result of their action upon cells, and secondarily, a result of their interactions between cells. The cell of a multicellular organism-man included-is also part of the tissue of an organ, and the organ is part of the whole organism.