Domain-driven design, extreme programming (XP), Scrum, and service-oriented architecture (SOA) are most widely accepted agile development practices. Today these agile development practices are combined with Design Thinking, Lean Startup, DevOps, microservice architecture (MSA), BizDevOps practices, etc. All these practices have a commonality. They all try to create the maximum value for the users of the software with minimal time and cost, and do so by releasing working software increments frequently to users, learn from the users' feedback and adapt the software to their needs. So these all can be regarded agile development practices.This book helps you understand the essence of agile development. The essential characteristics include value proposition, continuous and Lean delivery, validated learning, and software engineering competency. The essential characteristics of agile development are not peculiar. They are indeed very generic characteristics of properly reengineered business processes. They adopted common process reengineering patterns such as concurrent engineering, empowerment, job enrichment, flattened organization, Lean production, upstream shift, self-service, etc.This book goes beyond conceptual discussions about the essence of agile development. It shows practical examples of activities, work products and techniques that can be employed to perform agile development. More importantly it shows engineering principles that should be followed in producing work products in agile development. The reader will see how de facto standard languages such as ArchiMate, BPMN, UML and SoaML and their supporting tools can be used for agile requirement analysis and agile architecture design. Metamodels are presented which specify the dependencies among various elements in the models in different languages. It is shown that by following the engineering principles depicted in the metamodels, the contents of all kinds of work products produced in agile development become consistent and traceable with each other, thus cutting down the errors and the rework that would otherwise have been caused by inconsistency and incorrect transformations among the work products.Agile development based on new application platforms in the cloud (called aPaaS) is also illustrated using real examples. The currently fastest way of performing agile development using the so-called low code development platforms (a.k.a., high-productivity aPaaS or BizDevOps platforms) is illustrated, too. The book introduces the reader to a recent standard published by OMG called Essence, which provides a kernel and a language to describe and enact software engineering methods. It is shown that the Essence can be used to manage an agile development project in an adaptive manner so that the project plan can be daily adjusted based on the observed state of the project. It is also shown that the Essence can be used to assemble various agile development practices (such as Design Thinking, agile modeling, agile architecture design, XP, Scrum, Lean Startup, etc.) into a coherent methodology for a specific program or project. The Value-Obsessed Lean Framework (VOLF) is presented as a framework with which an integrated agile development method can be composed for a specific program or project. The VOLF ensures that all elements in the method are consistent and traceable.One of the most important things to learn from this book is to understand what knowledge, skills, practices, techniques and tools software project team members need to be trained in and become capable of, in order to be successful in performing an agile development project. This book provides a very comprehensive insights into agile development. It will make you rethink about agile development as a business strategy and as a business process that help you survive and succeed through digital transformations.