This book introduces the theories that address the process of measurement, the role of causality in measurement, and the meaning and interpretation of test scores. Philosophical and psychometric perspectives are explored as well as related problems and unresolved issues. As such, it appeals to researchers and advanced students interested in test validity in the behavioral, social, and educational sciences. Introductory chapters in each section provide an overview of existing literature for teaching purposes. The book examines how measurement is conceived of in both classical and modern perspectives on psychological testing. The importance of understanding the underlying conceptual issues as well as the practical challenges of construction and use is emphasized throughout. The extensive use of examples throughout the book brings the theoretical issues to life by showing how the theory is applied in actual testing situations. Boxed applications include critical thinking questions to engage readers. A few examples include: What is the difference between intelligence and IQ? Can people disagree on issues of value but agree on issues of test validity? Is it possible to ask the same question in two different languages? Organized into 4 sections, section 1 reviews the key theories. Section 2 examines the relation between measurements and theoretical attributes including both non-causal and causal relationships. Both the theoretical and practical consequences of choosing a particular measure is evaluated. Section 3 examines test validity theory as it relates to test score interpretation and the distinctions between different kinds of tests (e.g., attitude, knowledge, ability). Unresolved issues regarding meaning and measurement are also reviewed. The book concludes with a review of the common themes that run throughout the book as well as their interrelations and suggestions for future research. Intended for researchers and practitioners interested in test validity and/or developing tests in the behavioral, social, and educational sciences, this book also serves as a supplement for courses on psychometrics and/or testing and measurement, and/or or as a primary text in courses on measurement or test validity. Prior familiarity with technical testing material is not assumed.