Animal cells crawl on flat surfaces using lamellipodium – dynamic network of actin polymers and myosin motors enveloped by the cell membrane. Experimental analysis of the lamellipodial geometry, cell speed and actin dynamics in fish keratocyte cells combined with computational modeling suggested that steady crawling of the motile cells is based on a force balance between actin growth and myosin contraction. However, explanation of unsteady movements, especially of motility initiation and turning, remains elusive. I will present simulations of a 2D model of viscous contractile actin-myosin network with free boundary that, coupled with experimental data, suggests that stick-slip nonlinear adhesion is the key to understanding polarization and turning of the motile cells.