In-situ Measurements of Particles and CME Transients (IMPACT) - a suite of seven instruments that samples the 3-D distribution of solar wind plasma electrons, the characteristics of the solar energetic particle (SEP) ions and electrons, and the local vector magnetic field. IMPACT is one of the STEREO mission's four measurement packages whose principal objective is to understand the origin and consequences of coronal mass ejections (CME's). CME's are the most energetic eruptions on the Sun. They are responsible for essentially all of the largest solar geomagnetic storms. In order to understand and forecast CME's, we need 3-Dimensional images of them and of the ambient solar corona and heliosphere. Achieving this perspective requires moving away from our customary Earth-bound lookout point. To provide the images for a stereo reconstruction of solar eruptions, one spacecraft leads Earth and one trails Earth in its orbit about the Sun. Launch occurred on October 25, 2006.