Heavy oils, extra-heavy oils and tar sands are major players for the future of energy.They represent a massive world resource, at least the size of conventional oils. They are found all over the world but Canada and Venezuela together account, by themselves, for more than half of world deposits. They share the same origin as the lighter conventional oils, but their geological fate drove them into thick, viscous tar-like crude oils. Most of them result from alteration processes mediated by microbial degradation. They are characterized by a low content of lighter cuts and a high content of impurities such as sulfur and nitrogen compounds and metals ; so, their production is difficult and deployment of specific processes is required in order to enhance their transportability and to upgrade them into valuable products meeting market needs, and honouring environmental requirements.Although these resources are increasingly becoming commercially producible, less than 1% of total heavy crude oil deposits worldwide are under active development. The voluntarily wide scope of this volume encompasses geology, production, transportation, upgrading, economics and environmental issues of heavy oils. It does not pretend to be exhaustive, but to provide an authoritative view of this very important energy resource.