The observational approach has its roots in geotechnical engineering, and the recognition that geotechnical engineering decision-making often is confronted by significant uncertainties arising from an incomplete site understanding. The heterogeneity of real-world geological systems makes it imperative to manage uncertainty at the scale of individual geotechnical decisions. Likewise, the Triad approach recognizes that chemical contaminants are distributed heterogeneously (sometimes with, and sometimes without, clear spatial patterning) at hazardous waste sites. Therefore all data gathering, characterization, and remediation strategies must start from that assumption. As with the observational approach, the Triad relies on real-time decision-making as a way to manage decision uncertainty caused by heterogeneity in a cost-efficient way. Modifying data collection while work is underway permits refinement of site conceptual models and adjustments to designs and work activities to match actual site conditions as they are discovered and understood.