AstroSat Proposal Processing System (APPS) is a mission critical software solution designed and developed by the joint efforts of Indian Space Research Organization and the Inter-University Centre for Astronomy and Astrophysics. It facilitates the participation of global scientific community to submit scientific proposals for observations with India's first multi-wavelength space observatory. The software systematizes the proposal submission and review process before the successful proposed observations are scheduled to carryout scientific observations. This paper describes the overall architecture of the system, implementation stratagems, administrative aspects and security aspects of the software. The paper also describes the techniques adopted for seamless day-to-day operations in meeting the mission requirements. As a future direction, the paper summarizes the roadmap for development of a generic multi-mission proposal handling model from the experiences gained through APPS.