Airborne Molecular Contamination is a diverse field and requires many different approaches, Unlike particles, other most commonly known cleanroom contaminants, AMC's have various subclasses. Each of these subclasses, acids, basis, condensables, metallics, etc. many different analytical techniques. But, before even we reach the point of solving sources of excursion, we have to correlate defects and contaminants causing the said defects. The more obscure and infrequent the of defects are, the more difficult it is to find the source. In some cases by the time defects have manifested, the contamination has dissipated and if metrology for real-time analysis is not in place, it is near impossible to discover the root cause. This presentation uses example of such an excursion and goes through the process of "how to" find root cause of an excursion.