Comets have seduced societies since immemorial times. They are small bodies that can be observed when approaching the Sun while a coma and a tail of dust and gas are formed. As their orbits cross the one of the Earth, they can potentially hit the planet. Thus, the cometary impacts could have delivered organics to the early Earth. In this chapter the role of comets as contributors to the early evolution of Earth is reviewed. Also, the physical, chemical and mineralogical properties of comets are revisited here, together with the models designed to understand the complex set of processes on it. The implications for the origin and preservation of life in the planet are also discussed. © 2010 Nova Science Publishers, Inc. All rights reserved.