The chapters included within this book demonstrate that crime linkage is becoming an increasing part of police investigations and subsequent court proceedings (see Chapters 8-10). However, if this trend is to continue in a reliable and evidence-based fashion, the fundamental principles of crime linkage must be supported by empirical research. The current chapter, therefore, reviews the research studies that have amassed over the past decade or so that have tested the principles of behavioral consistency and distinctiveness which underpin crime linkage. This research has used a broad range of methodologies and the findings span a number of law enforcement jurisdictions across Europe, North America, Canada, Japan, and South Africa. The extant research will be critically considered to identify the disparity between how crime linkage is currently tested and how it is used during real-life police investigations. This sets the stage for subsequent Chapters 12 through 14 of the book that will describe how researchers are actively seeking to overcome these limitations and, therefore, provide a more solid empirical basis for the practice of crime linkage.