Organic electrochemical transistors (OECTs) efficiently translate ionic signals into electronic signals through an organic mixed ionic-electronic conductor (OMIEC) active layer. This OMIEC active layer typically consists of a semiconducting polymer with a conjugated backbone that enables electron transport and side chains that enhance ion motion. The performance metrics of these materials are intimately connected to their microstructure, necessitating the use of various microscopies to develop structure-property relationships that can be leveraged for high-efficiency OECTs. In this chapter, we take a close look at the imaging approaches used to interrogate OMIECs. We focus on optical imaging, scanning probe microscopy, electron microscopy, and X-ray imaging, describing the recent progress in the field of OMIECs, as well as future opportunities for these techniques to relate microscopic and nanoscopic observations to bulk performance.