This is a book about thermodynamics, not history, but it adopts a semi-historical approach in order to highlight different approaches to entropy. The book does not follow a rigid temporal order of events, nor it is meant to be comprehensive. It includes solved examples for a solid understanding. The division into chapters under the names of key players in the development of the field is not intended to separate these individual contributions entirely, but to highlight their different approaches to entropy. This structure helps to provide a different view-point from other text-books on entropy.