The cell shape and the rigidity of the extra-cellular environment have been shown to play an important role in the regulation of cellular processes such as cell division and differentiation; they have also been shown to play an important role in the development of tissues and in the onset of cancer and other pathological states and diseases. We study the physical principles that govern the response of cells to mechanical cues in their surroundings using elasticity theory and condensed matter physics. This includes phenomena such as cell adhesion, establishment of cell shape and cytoskeleton structure, elastic interactions of cells, patterning, aggregation and alignment of cells. Using computer simulations we also investigate the interaction of motor proteins and cytoskeleton filaments that produce forces and movements of cells including, stress-fiber assembly, axonal growth and blood platelet formation. For more information, see Table of Contents on the Left.