“As projects get more complicated, managers stop learning from their - perience. It is important to understand how that happens and how to change it…. Fallible estimates: In software development, initial estimates for a project shape the trajectory of decisions that a manager makes over its life. For ex- ple, estimates of the productivity of the team members influence decisions about the size of the team, which in turn affect the team’s actual output. The trouble is that initial estimates usually turn out to be wrong. ” (Sengupta, 2008) This book aims directly to increase the awareness among managers and practitioners that estimation is as important as the work to be done in so- ware and systems development. You can manage what you can measure! Readers will find in this book a collection of lessons learned from the worldwide “metrics community,” which we have documented and enhanced with our own experiences in the field of software measurement and estimating. Our goal is to support our readers to harvest the benefits of estimating and - prove their software development processes. We present the 5 ISO/I- acknowledged Functional Sizing Methods with variants, experiences, counting rules, and case studies – and most importantly, illustrate through practical - amples how to use functional size measurement to produce realistic estimates. The book is written in a practical manner, especially for the busy practitioner community. It is aimed to be used as a manual and an assistant for everyday work.