AbstractWe present a method to estimate electron densities in the ecliptic plane using the STEREO Heliospheric Imagers (HIs). The nature of Thomson scattering of photospheric light by solar wind electrons is such that visible‐light observations by HI provide a means to infer information about plasma density from afar. This is achieved using discrete tomography, whereby line‐of‐sight integrals from HI are used to estimate electron density in the heliosphere over a predefined grid. The technique is applied to the Earth‐impacting coronal mass ejection (CME) launched on 12 December 2008. The two vantage points afforded by STEREO are insufficient to reproduce the density structure of the CME in detail; however, the technique is successful in locating the presence of a density enhancement associated with the CME. When applied to consecutive images, we are able to use the technique as a means to track the CME through interplanetary space. From these observations we make estimates of the CME radial velocity and density profiles, the results of which are consistent with measurements made in situ by Wind at L1. This method is presented as a new approach to determine CME propagation from HI observations, one that avoids many of the assumptions of CME morphology and dynamics that are often applied when tracking CMEs in such data. We also expect that this technique will prove useful, and a test of the long‐standing heliospheric reconstruction technique employed by using single images over time (Jackson et al., 2006, https://doi.org/10.1029/2004JA010942) when views from many wide‐angle imagers become available.