This volume consists of lectures delivered at the Sixth Inter­ national Nathiagali Summer College on Physics and Contemporary Needs held at Islamabad from June 15 to July 2, 1981. The College used to be held at one of the scenic hill resorts of Pakistan, Nathiagali, hence the name of the College. The College was organized by the Pakistan Atomic Energy Commission (PAEC), under the patronage of the International Centre for Theoretical Physics (ICTP), Trieste, with a view to providing an opportunity for local physicists and physicists from developing countries for learning of the latest developments in various branches of physics. The University Grants Commission provided a financial grant for the participation of physicists from the universities of Pakistan. The College had 18 lecturers from 7 countries. The total participation in the College was by over 200 people from 18 different countries. There were 15 days of concentrated lecturing during the day followed by seminars and discussion sessions in the evenings. From its inception the College has had a broad-based, multi­ disciplinary emphasis. The purpose of the College has been to provide­ physicists in the developing countries with enough information in various branches of physics so that they can shift, or broaden, their field of research. In the poor countries, like Pakistan, physicists cannot always get facilities and opportunities to continue research in their original field of specialisation at a reasonable level.