Nickel plays both negative and positive roles in petroleum refining. Found as a naturally occurring trace element in many crude oils and bound in oil-soluble ligation, nickel concentrations range from a few to over 250 ppm. In the conversion of crude oils to transportation fuels, these concentrations generally cause problems. Nickel interferes with upgrading processes, severely deactivating many heterogeneous catalysts (such as FCC), catalyzing undesirable side reactions (such as carbonization), and devaluing any residual material in which it may be isolated (such as coke). Nickel also can play a positive role in refining, assisting in oil exploration, catalyzing upgrading processes (such as for N removal), and producing petrochemicals (such as hydrogenation production). With increasing environmental concerns, Ni also has been utilized to identify sources of unregulated releases of crude oil, such as spills and seeps. This paper presents these and other aspects of nickel chemistry with relationship in exp...