This chapter provides an overview of the authors’ efforts in vehicle trajectory exploration and motion planning based on PRONTO , a numerical method for solving optimal control problems developed over the last two decades. The chapter reviews the basics of PRONTO, providing the appropriate references to get further details on the method. The applications of the method to the constrained optimal motion planning of single and multiple vehicles is presented. Interesting applications that have been tackled with this method include, e.g., computing minimum-time trajectories for a race car, exploiting the energy from the surrounding environment for long endurance missions of unmanned aerial vehicles (UAVs) , and cooperative motion planning of autonomous underwater vehicles (AUVs) for environmental surveying.