Process intensification has become a very interesting approach, transforming current practices in chemical engineering and bringing forth new developments in equipment, processing techniques and operational methods. This development aims at more compact, safe, energy efficient and environmentally friendly process. Several unconventional processing techniques rely on alternative forms of energy. Chemistry under extreme and non-conventional conditions is an actively studied topic in applied research and industry. Alternatives to conventional synthetic procedures promise. enhancement of reaction rates, yields, selectivity and also bear promise of milder reaction conditions in chemical synthesis. During the last few decades, chemical application of ultrasound (US) and microwave (MW) irradiation has received a lot of attention and widespread research is going on in these areas. Significant enhancement of selectivities, rates and yields in chemical reactions has been achieved by means of US and MW irradiation. The popularity of US and MW irradiation as chemical laboratory techniques is rapidly growing. based on the number of publications, presentations and meetings. demonstrating their vast potential. Other less exploited methods are solar and plasma reactors. These, however, will not be touched in the present review, which focuses on the use of ultrasound and microwaves as sources of energy, mainly in catalytic applications. (C) 2004 Elsevier B.V. All rights reserved.