Containing the proceedings of the Fourteenth School on Operators in Functional Spaces, held in Novgorod, USSR, in the summer of 1989, this volume focuses on the spectral theory of differential operators. The emphasis is on estimates of the number of negative eigenvalues of elliptic differential operators and on the analysis of asymptotical distribution of eigenvalues. Three of the papers discuss distribution of eigenvalues, while the fourth studies scattering theory. The authors rank among the leading Soviet specialists in this area of research. Aimed at researchers in spectral theory of differential operators, scattering theory, and related problems in mathematical physics, this collection provides an excellent overview of some of the major problems in the field.