From the reviews: "Deformation theory is the study of the behaviour of a family of algebraic geometric objects, such as schemes or coherent sheaves, near a given element. . The book is recommended for advanced graduate students and researchers in algebraic geometry who want to learn deformation theory. . The book is clearly written, the abstract material is illustrated by examples where appropriate and there are exercises dealing with concrete geometrical problems at the end of each section." (Gábor Megyesi, The Mathematical Gazette, Vol. 96 (537), November, 2012) "In the development of algebraic deformation theory, a graduate text explaining the fundamentals of the theory had been lacking. So, eventually, somebody made the effort and wrote down the essentials. Happily this somebody is Robin Hartshorne . . The precise formulation and good language make the book capture the audience. . a fundamental text for anybody who wants to learn deformation theory . . Also, a lot of relevant references are included."­­­ (Arvid Siqveland, Mathematical Reviews, Issue 2011 c) "Deformation theory is a ubiquitous subject: From the Taylor expansion in Calculus to the deformation of Galois representations. . Since deformation theory could be considered a central topic in algebraic geometry . textbook where some of the main results and methods are collected in one place is certainly welcome. . inclusion of exercises and plenty of examples, make this book suitable for a course on this topic or for self-study, with the only prerequisite the now standard textbook on Algebraic Geometry by the same author." (Felipe Zaldivar, The Mathematical Association of America, March, 2010) "No doubt, this masterly written book gives an excellent first introduction to algebraic deformation theory, and a perfect motivation for further, more advanced reading likewise. It is the author''s masterful style of expository writing that makes this text particularly valuable for seasoned graduate students and for future researchers in the field. The list of 177 references at the end of the book, which the author frequently refers to throughout the text, is another special feature of the volume under review." (Werner Kleinert, Zentralblatt MATH, Vol. 1186, 2010)