Copepods, or more commonly referred to as the insects of the sea, have successfully colonised every aquatic environment, equating insects in terms of absolute and relative success. They represent up to 90-97% of the marine zooplankton biomass, but may also be conspicuous in freshwater systems. Copepods are the linchpin of aquatic foodwebs; they prey upon phytoplankton while simultaneously acting as a staple food for higher trophic level organisms, contribute to the vertical fluxes of carbon and sustain recycled production through the excretion of ammonia. Copepods can also signal possible climate change and are indicators of the effects of ocean acidification. They are also used as model animals for ecotoxicological and molecular studies, and might be adopted as control agents of disease vectors. Current studies are rapidly exploring multiple lines of research with an intended purpose to provide an up-to-date snapshot of some hot topics in the study of the distribution, biology and ecology of these ubiquitous crustaceans. The chapters collected in this volume, written by leading scientists in different fields of investigation, focus on a wide range of processes and scales, from global distribution to molecular investigations, witnessing the interest of the scientific community at different levels. These contributions point out the latest developments and case studies on a number of research issues, and will promote discussion and stimulate advances in each field of investigation. The editor is confident that readers will appreciate the contents of each chapter and will find in them inspiring suggestions for their research, or even just to satisfy their curiosity.