The NASA Controlled Ecological Life Support System (CELSS) Program has the goal of developing life support systems for humans in space based on the use of higher plants. The program has supported research at universities with a primary focus of increasing the productivity of candidate crop plants. To understand the effects of the space environment on plant productivity, the CELSS Test Facility (CTF) has been developed as an instrument that will permit the evaluation of plant productivity on Space Station Freedom. The CTF will maintain specific environmental conditions and collect data on gas exchange rates and biomass accumulation over the growth period of several crop plants grown sequentially from seed to harvest. To better understand the systems needed to support plants and maintain the environmental conditions required by CTF, an Engineering Development Unit (EDU) is being constructed at NASA Ames Research Center in the Advanced Life Support Division. The EDU will provide the means of testing and evaluating hardware solutions to CTF requirements.This paper reviews the CTF science and functional requirements, and provides a description of the EDU objectives, design approach, subsystem descriptions, and some of the technology tools employed in accomplishing the design.