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:...