The chapters in this new volume describe original research concerned with both theory and practice in measurement. The material originated in presentations made at a conference that brings together practicing professionals and theoreticians in diverse areas of measurement and related fields. The first group of papers embody the most important aspect of objective measurement — its application to diverse contexts and purposes. The examples included concentrate on two types of situations: performance assessment and criterion-referenced testing. The specific contexts range from writing assessment, to assessment of the professional development of teachers, to high school physics. The second group of papers are focused on an area of great importance in assessment today: the modeling of raters and judges in an assessment context. The volume concludes with papers that explore the domain of theory in measurement, characterized by an innovative approach to model-building. Even though they are categorized as theory papers, most are based on the complicated and interesting problems that arise in quite specific contexts. As such, they represent some of the most original and exciting developments in the field of measurement today ranging from multidimensional measurement to graph theory and clustering techniques. This volume presents original research concerned with the practice of measurement, and the theory of measurement. The words in the title, objective measurement indicate that the chapters are all related to a particular approach to the philosophy and practice of measurement. By objective measurement, we mean that, in a situation where a certain class of stimuli (for example, items) are used to measure certain individuals. The chapters in this book originated in presentations made at the International Objective Measurement Workshop (IOMW) devoted to exploring the interface between theory and practice in measurement. Based on the complicated and interesting problems that arise in quite specific contexts, they represent some of the most original and exciting developments in the field of measurement today.