The capacity to do work is called energy and it is essential for life processes. Any resource that can produce heat, power life, move objects and produce electricity is called an energy resource. Human energy consumption has grown steadily throughout history. Energy resources can be classified on a variety of basis like nature of their transaction, availability, etc. The most important classification is based on the conventionality in deriving the energy. According to this, energy resources are of two types – (i) Conventional energy resources (ii) Non-conventional energy resources. a) Conventional energy resources – Any energy resource that is limited in nature and non-renewable and has been used since a long time is termed a conventional energy resource. Examples of this include coal, petroleum, natural gas and electricity. b) Non-conventional energy resources- Any energy resource that is yet to be used on a large scale and can be naturally replenished on a human time scale is called non-conventional energy resource. Not only are these abundant in nature but also renewable. Examples of this include Nuclear or atomic energy, solar energy, wind power, biogas, tidal energy and geothermal energy.