The two viewpoints provided by the Heliospheric Imagers (HIs) on the STEREO spacecraft provide a means to infer the density distribution within solar wind structures by reducing the line of sight ambiguities that arise from a single viewpoint alone. Electron densities in the ecliptic may be estimated via the tomographic inversion of images from both spacecraft, which are assumed to represent Thomson scattered light from the photosphere. A grid is defined in the region of space common to the fields of view of both spacecraft, which allows the problem to be formulated as an inverse equation. The equation is then solved using an iterative algorithm to produce an array of densities that most closely fit the data. The method is applied to two Earth directed Coronal Mass Ejections (CMEs) that occurred during phases of the mission when the spacecraft were sufficiently separated. The resulting densities from successive images are then used to determine how the CMEs evolve through the heliosphere and to make estimates of their speed and density at Earth, which are compared with in situ measurements from the Wind spacecraft at L1.