Ascertaining the nature and scope of a priori knowledge is of key import to philosophical thought. Philosophers do not merely ponder this issue in order to solve an epistemological puzzle. Philosophers do also strive to understand the a priori in order to understand what doing philosophy amounts to. So what is the nature of a priori knowledge? And is philosophy an a priori endeavour? Starting off from such epistemological and meta-philosophical considerations, the papers here collected trace the import and role of a priori knowledge through the diverse disciplines of philosophy - from metaphysics to ethics, and from the philosophy of Language to the philosophy of science. The Contributors A. Burri, S. Psillos and D. Christopoulou, G. Ernst, H.-J. Glock., S. Häggqvist, P. Horwich, N. Kompa, C. Misselhorn, C. Nimtz, G. Rey, O. Scholz, W. Spohn, I. Stojanovic, N. Strobach, C. Suhm