This document summarizes the presentations and workshops of a conference on improving long-term monitoring (LTM) and remedial systems performance that was held in St. Louis, Missouri between June 8th to 11th , 1999. The conference was sponsored and developed by the Federal Remediation Technologies Roundtable, an interagency consortium dedicated to improving the availability of innovative technologies through technology evaluations, to developing (with industry) technical solutions to common contamination problems, and to working with States to overcome permitting barriers to the use of new technologies. The conference was designed to provide up-to-date information on LTM and systems optimization through presentations and topical workshops. The conference was convened to address the need to evaluate and assess the processes and practices related to monitoring and optimizing subsurface remedial performance and associated contaminant changes. These processes and practices are especially critical to in situ processes and natural attenuation strategies that require lengthy time frames to accomplish remediation. Implementation of these processes and strategies has resulted in high operation, maintenance, and monitoring costs which can impact the effectiveness, timing, and cost of remedial solutions. The objectives of the conference were to: (1) highlight successes and issues related to improving the performance of subsurface remediation technologies, (2) showcase practical approaches to cost-effective monitoring of remedial performance, and (3) identify research gaps and needs from current practice.