This document discusses using the Raspberry Pi's GPIO pins and interfaces like I2C and UART to build custom shields and expand the functionality of the Raspberry Pi. It provides an overview of the GPIO pins and interfaces available on the RPi and libraries for accessing them from code. It also describes using the KiCad electronic design software to design custom shields and discusses building prototypes of shields with components, PCB fabrication services, and assembly. Examples of navigation and smart I/O shields are presented to demonstrate expanding the RPi's capabilities through custom hardware.