This book is about developing apps for Android compatible mobile devices using the MIT App Inventor online development environment. MIT App Inventor projects can be in either standalone mode or use an external processor. In standalone mode, the developed application runs only on the mobile device (e.g. Android). In external processor-based applications, the mobile device communicates with an external microcontroller-based processor, such as Raspberry Pi, Arduino, ESP8266, ESP32, etc. In this book, many tested and fully working projects are given both in standalone mode and using an external processor. Full design steps, block programs, circuit diagrams, QR codes and full program listings are given for all projects. The projects developed in this book include: Using the text-to-speech component Intonating a received SMS message Sending SMS messages Making telephone calls using a contacts list Using the GPS and Pin-pointing our location on a map Speech recognition and speech translation to another language Controlling multiple relays by speech commands Projects for the Raspberry Pi, ESP32 and Arduino using Bluetooth and Wi-Fi MIT APP Inventor and Node-RED projects for the Raspberry Pi The book is unique in that it is currently the only book that teaches how to develop projects using Wi-Fi and Node-RED with MIT App Inventor. The book is aimed at students, hobbyists, and anyone interested in developing apps for mobile devices. All projects presented in this book have been developed using the MIT App Inventor visual programming language. There is no need to write any text-based programs. All projects are compatible with Android-based mobile devices. Full program listings for all projects as well as detailed program descriptions are given in the book. Users should be able to use the projects as they are presented, modifying them to suit their own needs.