Audits are an important and integral part of the EPA Hazardous Waste Engineering Research Laboratory (HWERL) Quality Assurance (QA) Program. As part of the overall QA program, audits are used to determine contractor compliance with quality assurance plans and to assess the overall quality of data collected during data gathering or data generation activities. Additionally, audits are useful in evaluating the procedures used in collecting and analyzing samples, in all facets of data quality, and in management of the activity. Often, an audit of a sampling or analytical activity will reveal a problem which otherwise may have gone undetected until the end of the project. In such cases, on-site corrective action recommendations are made. Four different types of audits are performed under the EPA HWERL QA Program: Management Systems Audits (MSA), Technical Systems Audits (TSA), Performance Evaluation Audits (PEA), and Audits of Data Quality (ADQ).