The important breeding objective in any crop plants is to improve grain yield potential and stability of the cultivars including in maize. Stability of performance becomes more important if crop is grown in less favorable environments. Tolerance to abiotic stresses is in general and drought through cold and heat tolerance in particular is essential in crop plants, as plant cannot alter an environmental stress external to itself. The effect and mechanism of all the three types of temperature stresses vary with respect to survival, growth and development and physiological processes. So during planning of crop improvement in open pollinated crops like maize for temperature tolerance through development of inbred and hybrids one must keep in mind regarding mechanism of temperature tolerance. The improvements in vegetative growth and yield performance of any of these crops will require selection of inbreds for hybrid development that have more efficient growth habits under unfavourable stresses conditions.