Planetary exploration has always been a very complex and risky engineering problem. Building on the past successes, more missions are currently being considered and investigated for the near future. One of the cornerstones of a successful lander-based planetary mission is the Entry, Descent and Landing System (EDLS) on which this work concentrates. The Entry, Descent and Landing sequence is introduced in the broader context of planetary exploration and the various systems involved in delivering safely a payload on the surface are presented. The literature review concentrates on a number of studies that emphasised the need to understand the whole EDL sequence to help designing better systems. It also introduces the reader to the current state-of-the-art both in term of EDLS technologies and design methods as well as identifying a need to design EDL systems in a more integrated and streamlined manner. From the conclusions of the literature review, the main drivers of the EDLS are identified and a new integrated computational framework, SPADES, is developed that provides analysis and design of entry systems. Current and new methods are built into the framework to size all the main EDL systems and provide realistic simulated scenario validated against past missions data. From this point onward, new mission data is generated in the form of the proposed 140 kg astrobiology-focused Vanguard Mars Mission, for which both a parachute and an inflatable-based EDLS is produced. In addition, an ESA mars lander concept is investigated, focusing on the scalability of powered landing systems up to the scale of the future Mars Sample Return vehicle. Finally, building on the case studies and additional mission scenarios, the influence of spatial and atmospheric variables on the performance of EDLS is systematically assessed to provide planet- wide access maps for specific EDL systems and characterise the impact of low-density atmospheric profiles on a number of mission parameters.