Tissue functions and properties are determined by the interactions between cells and their extra-cellular matrix. Cells, which are the active component of tissues, can sense mechanical stimuli provided by their surrounding matrix and respond by generating mechanical forces. These forces may then influence the reorganization of the matrix and begin a sequence of cell/matrix crosstalks that feedback onto themselves. Our understanding of these interactions and their influence on the evolution of soft tissue is thus critical to understand many phenomena in biological tissues, such as wound healing, or tissue morphogenesis. Experimental observations tell us that cell shape and size are intrinsically linked to its function, properties and motion. For instance, cell spreading area is known to control the expression of mitogen agents, and thus cell division. Inversely, cell elongation and orientation are linked to the properties and deformation of the surrounding extracellular matrix. The origins of these behavior can be traced down to the multiphasic nature of cytoplasm and the rich spectrum of chemo-mechanical processes occurring within it, including stress-fiber polymerization, structural deformation and mass transport. Such multiphasic media have traditionally been very well described by mixture theory based on a continuum representation of interpenetrating phases and their interactions. The objective of this chapter is to introduce such a formulation of cell mechanics in order to capture the fundamental mechanisms of cell contractility and its interaction with an underlying elastic substrate. As such, the cell is considered as a mixture of four different phases including a passive cytoskeleton, a distribution of contractile stress-fibers, the cytosol and a population of globular actin monomers dissolved in the cytosol. After discussing the general formalism based on balance laws and constitutive relation, the chapter introduces a numerical strategy, based on the extended finite element, to capture the contraction of cells on compliant substrate. Model prediction and comparison with experiments are finally provided and discussed.