The behavior of objects in the micro- and nanoworld are dominated by surfaces forces rather than gravitation or inertia. With the increasing relevance of micro- and nanotechnology, including lab-on-chip technology, microfludics and biochips, a profound understanding of the forces acting between surfaces is of prominent importance. We use atomic force microscopy (AFM) based techniques to study surface forces in a wide variety of systems. The so-called colloid probe technique allows us to sense the adhesion and friction forces between single particles, between particles and bubbles or monolayers at the air-water interface. Hydrodynamic forces at the nanoscale depend on the interaction between fluid molecules and solid surface and may lead to phenomena like boundary slip that are not accounted for in classical hydrodynamics. Forces across polymer melts play an important role in polymer-based composite materials. Under ambient conditions, capillary forces will occur at all nano- and microcontacts of hydrophilic materials. They often dominate the flow behavior of granular matter and are of critical importance for stiction effects in MEMS devices. The excellent force sensitivity of AFM allows us also to study forces between single molecules.