The topic of this lecture is long-term energy and environmental strategy compatible with low atmospheric CO2 concentrations. A global energy system model of optimization type, of which the objective function is the total system cost over time horizon, was used as a mathematical tool to derive the strategies. Under the great deal of uncertainty it is difficult to draw firm conclusions in which technological options are the greatest potential in achieving significant CO2 reduction in economical way. However, the simulation results in the energy system model indicate that massive CO2 mitigation targets can be satisfied under the large-scale deployment of innovative technology, highlighting essential roles for nuclear, renewables, efficient use of fossil fuel and CCS. The results support for pursuing multiple technologies simultaneously, rather than focusing on realistic technological options on the basis of current perceptions.