The theme here is plate boundary processes in a session chaired by Lidia Lonergan (Imperial College). The first talk, by John Dewey (University of Oxford), looks at plate boundary processes in the ancient (Caledonides of Connemara) and modern (California). It is followed by Richard Gordon (Rice University) examining non-rigid plate behavior, Andrew Long develops the case of transform development on the East Africa margin. The general discussion opens up the issue of ophiolite emplacement together with the stress state and earthquakes in the Indian plate. This set of presentations comes from a three-day meeting (3-5 October 2017) celebrating the 50th anniversary of the advent of the paradigm of plate tectonics. The meeting was convened to examine the state of the art and scope out new directions. There are 9 other sessions available to view together with the William Smith lecture by Dan McKenzie that concludes the meeting.