The only really sustainable form of energy is solar energy. The densely populated tropical countries can do very well from this completely sustainable energy source without any novel technologies. Most of the commercial energy used world-wide is supplied by using nonrenewable resources. Environmental damage-global warming, ozone hole, noise, chemical and radioactive waste-is due to high energy use. Environmental deterioration is a direct consequence of wealth generated and sustained by extremely cheap fossil fuel. The price of fossil fuel does not include the cost for the deterioration of the environment. We show in this paper that even a densely populated country like Bangladesh can attain a high standard of living by a proper utilization of solar energy. We suggest that poor tropical countries should mobilize their resources to develop solar technology.