Past spacecraft life support systems have used open-loop technologies that were simple and sufficiently reliable to demonstrate the feasibility of crewed spaceflight. A critical technology area needing development in support of both long duration missions and the establishment of lunar or planetary bases is regenerative life support. The subject of this paper is an ongoing study for a conceptual design of a Lunar-Base Controlled Ecological Life Support System (LCELSS) to support a crew size ranging from 4 to 100. An initial description of the LCELSS subsystems is provided within the framework of the conceptual design. The system design includes both plant (algae and higher plant) and animal species as potential food sources.