Planning of software projects has become complex and challenging more and more as time has passed. Software project managers are dealing with issues such as rapid change of customer demands, dynamic advancements of hardware platforms and intangible nature of software. Development effort estimation plays a vital role in software project planning because accurate estimation is needed when dealing with the complexity of software projects. Numerous researches have been conducted to improve the accuracy of development effort estimation in recent years and various methods have been proposed in this field. Extensive diversity of existing methods and the lack of unified investigation of which, have inspired us to write this book. We have tried to collect the most significant studies in terms of software development effort estimation based on the latest findings in this area. It is expected that this book can be an initial reference for researchers who want to work on the mentioned field. This initial reference might help researchers to have a simple overview of available effort estimation methods in software projects.