This volume contains papers presented to the 12th conference on Methods and Techniques of Mathematical Physics held on November 26 - December 2, 1989 at the Mathematisches Forschungsinstitut Oberwolfach.The papers mainly focus at direct and inverse boundary value problems arising in classical scattering theory, fluid mechanics, magnetohydrodynamics, and potential theory and they aim at interchanging mathematical tools and ideas among mathematicians, physicists, and engineers.