The problem of fouling is treated from three points of view; laboratory, plant, and economic. A laboratory apparatus, which simulates furnaces operating in high fouling circumstances, is discussed and test results are given for real situations. A fouling problem, which occurred in an actual operating furnace, is evaluated with respect to operating variables and chemical anti-foulants. Good lab-plant correlation is observed. The economic aspects of fouling are presented in terms of cleaning costs, lost profits, and energy consumption. An economic model is proposed, containing the key operating costs.