The problem of fouling is treated from three points of view; laboratory, plant, and economic.  A laboratory apparatus, which simulates a furnace operating in a fouling mode, is discussed and test results are given for real situations.  Two fouling problems, which occur in operating refinery furnaces, are evaluated with and without the presence of a chemical antifoulant.  The economic aspects of fouling are presented in terms of antifoulant effectiveness in reducing cleaning costs and lost profits caused by charge cutback, required for furnace cleanout.