For head mounted displays, like they are used in mixed reality applications, eye gaze seems to be a natural interaction modality. EyeMRTK provides building blocks for eye gaze interaction in virtual and augmented reality. Based on a hardware abstraction layer, it allows interaction researchers and developers to focus on their interaction concepts, while enabling them to evaluate their ideas on all supported systems. In addition to that, the toolkit provides a simulation layer for debugging purposes, which speeds up prototyping during development on the desktop.