This volume contains the lectures given at the second and third summer schools in physics held from May 19 - June 14, 1991 in Katmandu. It was organised by the BCSPIN. committee, comprising several Asian countries, Bangladesh, China, Sri Lanka, Pakistan, India, and Nepal (BCSPIN). The committee believes such joint activities should not only enrich physics research in the area but also promote better understanding among the neighbouring countries. It should also foster better scientific interactions between the east and the west and the north and the south.