Described herein are several embodiments relating to modular irrigation controllers. In many implementations, the irrigation controllers are modular in that various functional components of the irrigation controller are implemented in removable modules that when inserted into position within the controller, expand the capabilities of the controller. Also described are various different types of expansion modules that may be coupled to the modular controller, having as variety of functions and features, as well as related methods of use and configuration of these modules in the controller. In one implementation, an expansion module is provided that includes a microcontroller capable of sending and receiving data communications to and from a main microcontroller of the controller that executed irrigation programs, the data communications relating to an irrigation program.