The paper attempts to distinguish types of IDID (Internal Diesel Injector Deposits) with regard to the mechanism of formation, factors facilitating their formation and the method of identification. Investigations into their formation mechanism have been discussed in the paper. The results of the investigations into the chemical nature and probable source of these deposits have also been discussed. It has been pointed out that modern common rail fuel injection systems might be less tolerant to deposits due to tighter fitting tolerances, lower mass of parts and sophisticated injection strategies. Various approaches to IDID formation counteracting have also been discussed.