This Integrated Science Assessment (ISA) is a review, synthesis, and evaluation of the most policy-relevant evidence, and communicates critical science judgments relevant to the National Ambient Air Quality Standards (NAAQS) review. As such, the ISA forms the scientific foundation for the review of the primary (health-based) and secondary (welfare-based) NAAQS for particulate matter (PM). This ISA is composed of nine chapters. This introductory chapter presents background information, and provides an overview of EPA's framework for making causal judgments. Key findings and conclusions for consideration in the review of the NAAQS for PM from the atmospheric sciences, ambient air data analyses, exposure assessment, dosimetry, health and welfare effects, including judgments on causality for the health and welfare effects of PM exposure, are presented in Chapter 2. More detailed summaries, evaluations and integration of the evidence are included in Chapters 3 through 9. Chapter 3 highlights key concepts or issues relevant to understanding the atmospheric chemistry, sources, and exposure of and to PM following a 'source-to-exposure' paradigm. Chapter 4 summarizes key concepts and recent findings on the dosimetry of PM, and Chapter 5 discusses possible pathways and MOA for the effects of PM. Chapters 6 and 7 evaluate and integrate epidemiologic, controlled human exposure, and animal toxicological information relevant to the review of the primary NAAQS for PM. Health effects related to short-term exposures (hours to days) to PM are the focus of Chapter 6. Chapter 7 evaluates health evidence related to long-term exposures (months to years) to PM. Chapters 6 and 7 are organized by health outcome categories, such as cardiovascular or respiratory effects, and each section includes effects of the various types of PM studied. Chapter 6 also includes a summary and synthesis of the recent health evidence that uses systematic approaches to assess health effects of sources and constituents of ambient PM; most such studies have evaluated effects of short-term exposure. Chapter 8 evaluates evidence related to populations potentially susceptible to PM-related effects. Chapter 9 evaluates welfare effects evidence that is relevant to the review of the secondary NAAQS for PM.