ESDU 00016 is a practical guide to the current state of knowledge relating to fouling in crude oil preheat trains. It will be useful to crude oil refinery operators and to process engineers reviewing the impact of process conditions and equipment design changes on heat exchanger fouling rates. It was developed in collaboration with a group of refinery operators@ heat transfer equipment manufacturers and chemical suppliers. Fouling in crude oil distillation units costs the western world approximately 4500 million US dollars per annum and results in excess fuel being burnt in refinery furnaces. This burning excess fuel is a major source of CO2 and other greenhouse gas emissions@ with a highly significant global environmental impact. The economic and environmental significance of crude oil fouling is described. The various types of fouling and its effect on exchanger preformance are reviewed. Factors influencing the build-up of fouling are identified and discussed. Practical recommendations are made for fouling mitigation and control@ based on the experience of the operators and process engineers. Opportunities to reduce fouling through the use of mechanical@ chemical and operational means and by cleaning are discussed in depth. Fouling prediction methods are reviewed and new techniques for modelling heat exchangers and preheat trains are described. The use of a published equation for the prediction of crude oil fouling thresholds is illustrated by ESDU's shell-and-tube heat exchanger software. The EXPRESS program can be used to identify designs that will operate below fouling thresholds. The REVISE program allows the performance and operability of existing shell-and-tube heat exchangers to be assessed and shows how simple design modifications affect the likely fouling performance. A comprehensive bibliography reflecting the state-of-the-art in crude oil fouling knowledge is presented.