Sunscreens are universally recommended by dermatologists not only to prevent the immediate effects of overexposure to sunlight but also to prevent skin cancer. While the former goal is immediately evident, the latter remains an unproven hypothesis and is a topic of some controversy. Recent epidemiological studies suggesting a correlation between increased use of sunscreens over the past two decades and the rise in skin cancer have led to the question whether sunscreens applied to skin may be undergoing photoreactions, the effects of which are elaborated many years later. By addressing the key questions, this book advances the field of sunscreen photobiology and provides the reader with an unbiased perspective on this important field.