Hydrothermal nickel systems are one of the most puzzling mineral systems that involve trace element mobility in the Earth's crust. Nickel is an abundant element in the crust. However, the processes that are involved in its mobility and accumulation at ore scale are challenging. A new empirical-stochastic model is required to gain a deeper insight into these systems. This Special Issue presents several studies on hydrothermal nickel accumulations world wide aiming to stimulate the debate on this intriguing deposit type. © 2012 Elsevier B.V.