This paper will explore assessment response processes and their relationship with process data. It is often argued that assessment reponse processes are internal, cognitive processes or mental operations that we are not able to directly access (Hubley and Zumbo, 2017; Bruchner and Pellegrino, 2016; Ercikan, Guo and He, 2020). The idea is that with process data we capture ‘traces‘ of response processes, rather than the processes themselves (Ercikan, Guo and He, 2020). The distinction has parallels with the idea that in test scores we can only attempt to make valid interences about traits and abilities. That we cannot ever hope to have direct or unmediated access to the trait or ability that we seek to investigate. However, this paper argues that the cognitivist view of response processes as internal mental operations seems unnecessarily restrictive, since it does not take on board more contemporary theories of embodied and extended cognition. This paper will present, discuss and extend working definitions and theoretical perspectives on response processes, and multimodal process data. By exploring ideas of traces and sensors, the paper will consider our points of contact with response processes, what can be captured and observed, and how we can make valid, reliable, and useful inferences.