Significant progress has been made in the design and execution of ground-water investigations for site assessments, the selection of remediation options and long-term monitoring of corrective action operations. Improved reliability and cost-effectiveness in ground-water studies can result from the recognition of both the complexity of the subsurface environment and the need for simple, integrated monitoring approaches. Continued progress in the methodologies of investigation will be enhanced by improvements in conceptualization of subsurface conditions.