Cell migration plays a vital role in many biological processes and the mechanism for both single cell and collective cell migration remains unclear. It is a complex multi-step procedure, including signal receiving, signal processing, force generation and when considering multicell migration, cell-cell interaction. Thus it inspires many scientists in biology as well as interdisciplinary areas to study this topic. This dissertation includes the study of cell migration from different aspects and on different scales, using quantitative models. I write them with a sequence from the smallest scale to the largest. In Chapter 2, I study the kinetics of the activation of G-protein-coupled receptors used in chemotaxis trying to explain the two activation rates observed in the experiments. In Chapter 3, I study single cell chemotaxis, focusing on the signaling networks to explain the memory effect observed in the experiments. In Chapter 4, I study the collective cell chemotaxis taking cell-cell communication into consideration, trying to find the possible minimal network topologies for the signal processing. Finally, in Chapter 5, I study multi-cell migration on a much larger scale with thousands of cells and use a simplified model which focuses on the different kinds of forces applied on the cells to study the relation between single cell properties and large scale behaviors shown in the multi-cell migration. All these works provide some new knowledge on part of the mechanism controlling cell migration.