Solar Coronal mass ejections (CMEs) are large-scale ejections of plasma and magnetic field from the corona, which propagate through interplanetary space. CMEs are the most significant drivers of adverse space weather on Earth, but the physics governing their propagation through the Heliosphere is not well understood. This is mainly due to the limited fields-of-view and plane-of-sky projected nature of previous observations. The Solar Terrestrial Relations Observatory (STEREO) mission launched in October 2006, was designed to overcome these limitations. In this thesis, a method for the full three dimensional (3D) reconstruction of the trajectories of CMEs using STEREO was developed. Observations of CMEs close to the Sun (<15 R) were used to derive the CMEs trajectories in 3D. These reconstructions supported a pseudo-radial propagation model. Assuming pseudo-radial propagation, the CME trajectories were extrapolated to large distances from the Sun (15-240 R). It was...