This book is a monograph on unitals embedded in finite projective planes. Unitals are key structures in projective planes, and have connections with other structures in algebra. They play a significant role in the classification of finite planes and provide a link between groups and geometries. There is a considerable number of research articles about unitals, and there also exist many open problems. This book is a thorough survey of the research literature on embedded unitals which collects this material in book form for the first time. The book is aimed at graduate students and researchers who want to learn about this topic without reading all the original articles. The primary proof techniques used involve linear algebraic arguments, finite field arithmetic, some elementary number theory, and combinatorial enumeration. Some computer results not previously found in the literature also are mentioned in the text. The authors have included a comprehensive bibliography which will become an invaluable resource. In addition, group theoretic characterizations of classical and Buekenhout-Metz unitals are catalogued and summarized in an appendix.