Mechanical interaction between cancer cells and their microenvironment plays a central role in the progression of tumors. In vitro models based on biopolymer networks have been successfully employed to simulate the 3D extracellular matrix (ECM) of tumors. In this review, we focus on type I collagen gel. We describe the hierarchical structural and mechanical properties of type I collagen ECM. We demonstrate that corresponding to the scales of adhesion sites, single cells and cell colonies, the mechanics of the ECM is dominated by single fibers, fiber clusters and rheology of the whole fiber network. In the end, we discuss the limitations of reconstituted type I collagen as in vitro ECM.