Cell-matrix junctions such as focal adhesions are directly connected to a network of contractile proteins such as actin and myosin. When mechanical forces pull the junctions away from a rigid extracellular matrix, the cell forms additional focal adhesions to withstand the tension. A softer ECM generates less resistance, allowing the cell to respond according to the stiffness of ECMs in different tissues. Such mechanosensing depends on junctional proteins such as talin and fibronectin that change their conformation when the junction is stretched. As actin filaments get pulled by myosin, talin unfolds, exposing vinculin-binding sites. Vinculin recruits additional actin filaments to strengthen the junction. On the ECM face, tension applied across the length of fibronectin during fibril matrix assembly exposes hidden binding sites for collagen. Collagen binding reinforces the junction allowing cells to respond to mechanical forces from their environments.