In this review paper, we report the molecular structure, origin, natural occurrence and variety of diamondoid hydrocarbons in petroleum fluids and other fossil fuels. In addition, we present the role of diamondoids as geochemical tools for petroleum characterisation, ways diamondoid molecules are used to play an important role in petroleum evaluation, and may be used to assess origin, extend of biodegradation and thermal maturity, as well as to identify the occurrence of petroleum. The diamondoid-based diagnostic ratios in environment science as well as diamondoid-based diagnostic ratios for petroleum maturity parameters are also presented and discussed. Such diamondoid-based parameters may be used to assess oil source, maturity and biodegradability, providing supporting information, especially when traditional evaluation data are not available or are difficult to interpret. We also report applications and problems associated with diamondoids in petroleum exploration and production. That is the role of diamondoids in petroleum and natural gas production fouling as well as separation, detection and measurements of petroleum diamondoids. We realise that the presence of diamondoids in petroleum has become much more than a chemical curiosity since it has advanced to be a resourceful instrument in petroleum science, biomedicine, materials science and an important family of molecular building blocks in nanotechnology.