This volume contains papers based on some of the talks given at the NSF-CBMS conference on 'The Geometrical Study of Differential Equations' held at Howard University (Washington, DC). The collected papers present important recent developments in this area, including the treatment of nontransversal group actions in the theory of group invariant solutions of PDEs, a method for obtaining discrete symmetries of differential equations, the establishment of a group-invariant version of the variational complex based on a general moving frame construction, the introduction of a new variational complex for the calculus of difference equations and an original structural investigation of Lie-Backlund transformations.The book opens with a modern and illuminating overview of Lie's line-sphere correspondence and concludes with several interesting open problems arising from symmetry analysis of PDEs. It offers a rich source of inspiration for new or established researchers in the field. This book can serve nicely as a companion volume to ""Selected Topics in the Geometrical Study of Differential Equations"", by Niky Kamran, in the ""AMS"" series, ""CBMS Regional Conference Series in Mathematics"".