Geršgorin and His Circles This book studies the original results and their extensions of the Russian mathematician S.A. Geršgorin who wrote a seminal paper in 1931 on how to easily obtain estimates of all n eigenvalues (characteristic values) of any given n by n complex matrix. Since the publication of this paper there has been many newer results spawned by his paper and this book will be the first which is devoted solely to this resulting area. As such it will include the latest research results such as Brauer ovals of Cassini and Brualdi lemniscates and their comparisons. This book is dedicated to the late Olga Taussky Todd and her husband John Todd. It was Olga who brought to light Geršgorins paper and its significance to the mathematical world. The level of this book requires only a modest background in linear algebra and analysis and is therefore comprehensible to upper level and graduate level students in mathematics