Cells in vivo migrate in a complex microenvironment and are subjected to varying degrees of physical confinement provided by neighboring cells, tissues, and extracellular matrix. The molecular machinery that cells utilize to migrate through confining pores or microtracks shares both similarities and differences with that used in unconfined 2D migration. Depending on the exact properties of the local microenvironment and cell contractile state, cells can adopt distinct phenotypes and employ a wide array of mechanisms to migrate efficiently in confined spaces. Remarkably, these various migration modes are also interconvertible and interconnected, highlighting the plasticity and inherent complexity underlying confined cell migration. In this book chapter, an overview of the different molecular mechanisms utilized by cells to migrate in confinement is presented, with special emphasis on the extrinsic environmental and intrinsic molecular determinants that control the transformation from one mechanism to the other.