This book is a collection of five chapters discussing catalase and its various applications in different fields. Chapter One discusses the cardiovascular effects of a catalase blockade of H2O2 in normotensive and hypertensive rats. Chapter Two reviews daily and seasonal changes in activity of catalase in littoral macroalgae on the Barents Sea, as well as the influence of abiotic factors in the aforementioned. Chapter Three is an overview of plant catalases under abiotic stress. Chapter Four analyses aspects of the blood catalase in the human body, and finally, Chapter Five discusses the inhibitory effect of drugs on catalase activity.