In the last decades, the Automated Driving (AD) has become a cornerstone in modern research on transportation, since it can significantly improve urban mobility, reducing traffic congestion, while lowering pollution and energy consumption, and enhancing road safety by avoiding human failures that most of the time are crashes source. Specifically, the present work focuses on SAE Level 4 (i.e. High Driving Automation) autonomous vehicles (AVs) that travel within urban roundabouts, proposing new strategies to handle the Motion Planning (MP) in such complex scenarios. The state-of-the-art underlying this thesis introduced a three-layers structure for the MP architecture, given by a Global Path Planner (GPP), a Local Trajectory Planner (LTP) and a Nonlinear Model Predictive Control (NMPC) controller, merging together in a complete AD system.