Fouling in crude preheat train heat exchangers in refineries is a complex phenomenon. Crude oil fouling undergoes different mechanisms at different stages of preheating. Understanding the fouling mechanisms is essential in formulating appropriate fouling mitigation strategies. The use of the concept of threshold fouling conditions is one of the approaches for mitigating fouling through operating conditions. In this study, an attempt has been made to review the various fouling models available in literature, their advantages and limitations.