Kirstine Berg-Sørensen is associate professor in biological physics at the Department of Physics, Technical University of Denmark. She was educated at the University of Aarhus, with a 1-year stay with Jean Dalibard at the Ecole Normale Superieure in Paris, France, during her Ph.D. studies. In her Ph.D. work and first postdoctoral experiences she concentrated on quantum mechanical and semiclassical models for laser cooling and the physics of cold atoms, as a postdoctoral fellow with Lene Hau, Rowland Institute for Science, Cambridge, MA, USA, and Chris Pethick, NORDITA, Copenhagen, Denmark. Later, her research activities became focused on the physics of biological systems; triggered by some of the first reports on the use of optical tweezers in single molecule biophysics. Her own research in biological physics has been concentrated on simple models and precise data analysis for single molecule experiments, both under in vitro and in vivo conditions, with main emphasis on optical trapping by optical tweezers and lately, also optical stretchers. Her current work is both experimental and theoretical, related to optical stretchers.