Process intensification (PI) is the strategy of making significant reductions in the size of a chemical plant in order to achieve a given production objective. Innovations in catalytic reactors, which constitute the heart of such process technologies, are often the preferred starting point. Trickle-bed reactors are multifunctional reactive systems (catalysts, reactors, etc.), and allow a unique way of achieving PI in the chemical process and refining industry. In this millennium, process intensification in trickle-bed reactors is essential to meet the ultra low specification of the transportation fuel. The process intensification by operating at elevated pressure and temperature, and better understanding of flow behavior (which also helps for better design and scaleup), inducing pulse and operating in countercurrent mode is discussed here. Recommendations have been made for the future research in this area. (c) 2005 Elsevier Ltd. All rights reserved.