In order to advance entry guidance technology two different research areas have been explored with the objective of increasing the reachable landing area and the landing accuracy for future Mars missions. Currently only the northern hemisphere of Mars is available for landing due to its low elevation. Only low elevation landing sites have the necessary atmospheric density to allow landing using current Entry, Descent and Landing (EDL) technology. In order to reach most of the Ancient Highlands, the majority of the southern hemisphere, advanced EDL technology is needed in multiple fields, including entry guidance. The first research area is the definition and applications of reachable and controllable sets for entry. The definition of the reachable and controllable sets provides a framework for the study of the capabilities of an entry vehicle in a given planet. Reachable and controllable sets can be used to comprehensively characterize the envelope of trajectories that a vehicle can fly, the sites it can reach and the entry states that can be accommodated. The sets can also be used for the evaluation of trajectory planning algorithms and to assist in the selection of the entry or landing sites. In essence, the reachable and controllable sets offer a powerful vehicle and trajectory analysis and design framework that allows for better mission design choices. In order to illustrate the use of the sets, they are computed for a representative Mars mission using two different vehicle configurations. The sets characterize the impact of the vehicle configuration on the entry capability. Furthermore, the sets are used to find the best skip-entry trajectory for a return from the Moon mission, highlighting the utility of the sets in atmospheric maneuvers other than entry. The second research area is the development of the components of an entry guidance algorithm that allow high elevation landing and provide as well high landing accuracy. The approach taken follows the acceleration guidance approach successfully used in missions like the Space Shuttle. Based on this approach, a new planner has been developed that is computationally fast and provides trajectories with both high final altitude and high control authority. A new tracking law based on nonlinear predictive control has been developed to track the reference trajectory. The new tracking law accounts for control saturation, which is a common feature of trajectories in the thin Martian atmosphere with a low lift vehicle. Finally, a final position guidance algorithm that aligns the vehicle's heading towards the target and provides altitude and range control, has been developed. The new guidance algorithm provides increased accuracy as well as higher final altitude compared to an entry guidance algorithm representative of the current capabilities.