Studying the ocean at night can be difficult. Yet this is precisely the time when many fish are most active. Scientists have found that many important processes occur at night, such as spawning, larval settlement, migrations, feeding, and more. Many organisms rise toward the surface, creating massive pulses of biodiversity and biomass. In nearly every aquatic environment, from open waters to coral reefs, what one observes by day can be quite different from what is happening after the sun sets. At the same time, in polar seas and at great depths, “night” can span, months, years, and beyond. Fish and fishers in these dark systems have adopted tactics and strategies that take advantage of low-light conditions and their study may offer solutions to problems in warmer, shallower habitats.