Hydrocracking is widely practiced by the refiners to convert heavy petroleum feedstocks into desired lighter products. There are many process variables and catalyst formulations for optimization of hydrocracker performance with respect to yield pattern, products' quality and catalyst life. A user-friendly steady state hydrocracker model (HC-MOD) has been developed based on novel continuum theory of lumping approach for simulating hydrocracking kinetics and heat effects in a commercial hydrocracker. The model formulation includes skewed Gaussian type primary yield distribution function to describe yield and selectivity of the hydrocracking components. The model provides excellent prediction of commercial units' performance, as it incorporates hydrocracking kinetics of paraffins, naphthenes, and aromatics. The capabilities of HC-MOD include prediction of products' yields and qualities, reactor bed temperature profile, estimation of chemical hydrogen consumption, etc. In the present paper, case studies from operating refineries are presented where HC-MOD has been successfully applied for optimization and troubleshooting, leading to better performance. The cases involve: (i) solving low conversion problem in a two stage hydrocracker, (ii) catalyst selection for maximum middle distillates, and (iii) increase in middle distillates selectivity through optimization of operating conditions. The paper also describes the knowledge-based solutions, which have led to the significant benefits to the operating refineries. (C) 2004 Elsevier B.V. All rights reserved.