This book contains invited lectures and shorter contributions to the workshop 'Mathematical, Theoretical and Phenomenological Challenges Beyond the Standard Model: Perspectives of the Balkan Collaborations' (BW2003), which was held in Vrnjacka Banja, Serbia, 29 August - 3 September. It was one of the first high-level HEP workshops in the South-East European region after many years, and the papers give a clear perspective on the scientific potential of this area.The contributions cover topics and problems under research in high-energy particle physics and quantum field theory, in particular: string theory and M-theory; grand unification; quantum gravity and cosmology; standard and noncommutative quantum field theory.Some papers concern more mathematical topics, such as the integrability topology of gauge groups and operator matrices.The contributors are leading international scientists, such as G Arutyunov, K Behrndt, G Cardoso, M Chaichian, G Lazarides, D Luest, N Kaloper, G Senjanovic, and J Wess, with an emphasis on researchers from South-East Europe, including B Bajc, A Mikovic and B Sazdovic.The proceedings have been selected for coverage in:• Index to Scientific & Technical Proceedings® (ISTP® / ISI Proceedings)• Index to Scientific & Technical Proceedings (ISTP CDROM version / ISI Proceedings)• CC Proceedings — Engineering & Physical Sciences