This book is about the chemistry of phosphate monoester hydrolysis catalyzed by enzymes called phosphatases, focusing on their chemistry at a molecular level, i.e. their structure, function, and mechanism of action. At present, six classes of phosphatase are well characterized: (i) alkaline phosphatases, (ii) purple acid phosphatases, (iii) high molecular weight acid phosphatases, (iv) low molecular weight acid phosphatases, (v) phosphotyrosine phosphatases, and (vi) serine/threonine phosphatases. This book presents a comprehensive overview of phosphatases, comparing and contrasting Nature's different solutions to the problem of phosphate monoester hydrolysis. This is the first thorough survey and comparison of these different types of phosphatases. It focuses on chemistry, not biology.