Single molecule methods are essential for a thorough understanding of complex biological processes. They allow real time observation of molecular machines at work and their specific manipulation. Results of such experiments yield new insights into problems from fundamental physics at the nano-scale to the development of new drugs. Methodologically we focus on single molecule multi color FRET, optical tweezers and single molecule force spectroscopy based on AFM. The multidisciplinarity of our research requires close collaborations with colleagues in biology, biochemistry, chemistry, medicine, biotechnology and physics.