Key processes in Solar-Terrestrial Physics deals with a nice selection of key phemena concerning Solar-Terrestrial relations. During the week of October 4-9, 2009, about 160 participants from 19 countries met at the Itamambuca resort area of Ubatuba, Sao Paulo, Brazil to discuss the influence of solar variability on geophysical and heliospheric phemena at a conference organized by the International Living With a Star (ILWS) Program of NASA and by the National Institute of Space Research (INPE) of Brazil. Five of the invited review talks of this Conference are being published in this special issue, plus one (on magnetospheric reconnection) especially invited to cover a missing important subject within the Solar-Terrestrial physics domain. Previously published in Space Science Reviews journal, Vol. 158/1, 2011.