Recent studies on Rice focus on improving biomass partitioning and increasing photosynthetic efficiency by suppressing photo respiration and reduction of maintenance respiration in traditional C-3 rice cultivars, and inducting C-4 mechanism in new cultivars as means to maximize rice productivity where water and nutrients are not limiting. To target and achieve the set productivity in today’s cultivars one should understand the relations among climate, water. nutrients and soil factors.The productivity is quantitatively determined by (1) number of panicles, (2) spikelet fertility/sterility (3) number of grains per  panicle and (4) grain weight. The limits of productivity in situations posing different constraints and ways of approaching nearer to the targets are discussed.