The lectures here reported were first delivered in August and September, 1965, for the Department of Mechanical and Aerospace Engi­ neering at syracuse University, New York under the sponsorship of the New York State Science and Technology Foundation. Lectures 1-6 and 22-23 are revised from a version prepared by Professor Kin N. Tong on the basis of a transcription of the lectures, kindly provided by Professor S. Eskinazi. The remainder of th~ text has been written out afresh from my own notes. Much of the same ground was covered in my lectures to the Austra lian Mathematical Society's Summer Research Institute at Melbourne in January and February, 1966, and for the parts affected the text conforms to this latter presentation. I am grateful to Professors C.-C. Wang and K. N. Tong for criticism of the manuscript. These lectures constitute a course, not a treatise. Names are attached to theorems justly, to the best of my knowledge, but are not intended to replace a history of the subject or references to the sources.