This book is designed to develop both server and client for an application. We have used the Kotlin language for both the server and client sides. In this book, Spring will be the server-side application, and Android the client-side application. Our primary focus is on those areas that will be able to help a developer develop a secure application with the latest architecture. This book describes the basics of Kotlin and Spring, which will be of benefit if you are unfamiliar with these platforms. We also designed the chapters for implementing security and database in a project. This book delves into the use of Retrofit for handling HTTP requests and SQLite Room for storing data in an Android device. You will also be able to find a way of how to develop a robust, reactive project. Then, you will learn how to test a project using JUnit and Espresso for developing a less bug-prone and stable project.