Publisher Summary This chapter focuses on a device driver, which is an extension of the free BSD kernel that implements a standard software interface to hardware. Device drivers consist of data structures and a fixed set of functions provided by the device driver writer. The kernel calls driver functions in response to conditions— such as driver load, power management event, device interrupt, or an application requesting a service. To develop a device driver, an understanding of the related kernel data structures and driver method functions is needed. The chapter includes driver environment, device driver kernel data structures, driver method functions, steps for developing a FreeBSD, and the PCIO-DIO24 device driver. Major data structures, available tools, and a description of the driver build environment including the driver developer, the necessary information to implement a functioning FreeBSD device driver are also discussed.