Crop physiologists have often been suprised at the lack of correlation between single leaf photosynthesis and yield. Conversely, canopy photosynthesis is highly correlated with yield. This difference should not be suprising because canopy photosynthesis is an integrated measure of 3 of the 4 physiological components of yield. These 4 components are: 1. absorption of PPF by photosynthetic tissue, 2. carbon fixation (photosynthesis), 3. carbon use (respiration), and 4. carbon partitioning (harvest index). We have used this model to analyze the short and long-term consequences of environmental changes on plant productivity. The individual components of the model can be measured separately. This paper reviews the model and focuses on the third component: carbon use efficiency.