Less than ten years ago, it was discovered that viruses are at least 1,000 times more abundant in the marine environment than previously thought. This illustrated monograph reviews the fast-growing body of knowledge on these micro-organisms, and in particular their potential impact on Mediterranean biogeochemistry, their influence on bacterial diversity, on food web structure, their relation with anthropogenic impacts, etc. This volume, offering original syntheses on viral dynamics and diversity, and providing a review of key research questions in the field, should prove of high value to many marine scientists.