The main aim of this project was to develop novel concepts for miniaturization of bioanalytical techniques for investigating biomolecular interactions. We used optical tweezers to selectively address individual biological objects in microfluidic channels. A general introduction of applications of optical tweezers and microfluidics is given in chapter 1. Theoretical concepts related to optical trapping and microfluidics are reviewed in chapter 2, followed by a detailed description of the instrumentation in chapter 3. In chapter 4, ligand-receptor interactions are studied under physiological conditions: whole cells or native vesicles carrying in their membrane the protein of interest are immobilized first in the laser trap inside a microfluidic channel, then the reaction is initiated by changing the solution in the region around the trap. In chapter 5 and chapter 6 respectively, surface-modified polystyrene beads are used to study ligand-receptor interactions and DNA hybridization. The examples of these different systems studied at single cell or cell-mimicking system level demonstrate the potential to perform optical trapping in microfluidic channels and thereby constitute the basis for future applications towards parallelization and automatization of (bio)analytics. Chapter 7 and chapter 8 deal with two applications of microfluidics and optical trapping separately. Chapter 7 describes an approach for downscaling cell culturing by using microfluidic chips instead of the usual cell culturing dishes. Chapter 8 deals with an application of optical tweezers, as a tool for micromanipulation at sub-cellular level. The last chapter closes the present report by giving an overall discussion of the results and outlook for further projects.