This book is a state-of-the-art review on recent advances in flow cells for electrochemical energy systems. The book includes an introduction to flow cells, proton exchange membrane fuel cells, photocatalytic fuel cells, organic flow batteries, redox flow batteries, microfluidic flow cells, as well as electrolysis cells for CO2 and nitrogen reduction. The book provides an essential reference for professors, researchers, and policymakers globally in academia, industry, and government.