The operations of the Space Station Freedom (SSF) Environmental Control and Life Support System (ECLSS) are determined by the configurations of the ECLSS subsystems. The configurations of the ECLSS subsystems are in turn determined by the NASA budget approved by Congress and the amount of money allocated to the SSF program. This paper uses the concepts of top-down techniques and the theory of optimization to present the mathematical formulations for determining the optimal configuration and optimal operation of the SSF ECLSS, Sbased on different scenarios. The formulations can be used to determine the optimal numbers of units for the ECLSS processors to be configurated, their recovery efficiencies, and the crew potable and hygiene water uses. Examples are illustrated.