In the course of this dissertation applications of spatial light modulators were studied. Spatial light modulators are liquid crystal micro devices which can manipulate amplitude and phase of an incident light beam. The first part of this thesis presents how spatial light modulators can be implemented in conventional optical microscopy set-ups for Fourier filtering in imaging applications. The second part of the thesis is dedicated to a further development of a conventional single beam holographic optical tweezers set-up. Furthermore we also developed a combination of acoustic (ultrasonic) and optical trapping, which enables stable trapping of large particles, or even highly motile micro-organisms in 3D.