TMDLs, or total maximum daily loads, are required under the Clean Water Act, Section 303(d), for waterbodies that do not attain water quality standards. The objective of this research was to review the existing TMDL process and to develop an improved design for TMDL development where improvements are needed. This objective was accomplished through: (1) Identifying and verifying problems with the existing TMDL program; (2) Formulating a range of recommended improvement options on specific topics that address identified weaknesses; and (3) Providing case studies examples that highlight these recommended changes to improve the TMDL process. The research team identified ten specific areas for improvement, based on a review of approved TMDLs, a survey of state programs, and an in-depth review of specific case study examples. These topics relate to specific steps and/or methods employed in the TMDL development process. They include: estimating background pollutant loads; quantifying nonpoint source loads using simple methods; incorporating urban wet weather sources; selecting critical conditions; guiding principles for modeling; estimating the TMDL margin of safety; approaches to the allocation of loads; linking best management practices to load reduction; implementation planning, tracking, and adaptation; and adaptive watershed management. Each topic is addressed in a separate chapter of the report, with a discussion, recommendation of improved approaches where appropriate, and specific examples to help guide water quality professionals when developing or reviewing TMDLs.