Quality data are necessary to test research questions about psychopathy. This chapter describes the research design of the Incarcerated Serious and Violent Young Offender Study (ISVYOS). The ISVYOS was initiated in 1998 and has been ongoing ever since. In this chapter I describe the ISVYOS that was used in the book, procedures for interviewing youth and collecting data in adulthood, control variables that were accounted for in the different analyses in this book, and the different measures of psychopathy that were available. This book focuses on 535 incarcerated youth who received an interview and rating on the PCL:YV. Many of these youth received psychopathy assessments using additional measures. To give readers a better sense of how psychopathy manifests in youth, I describe procedures for rating the PCL:YV and discuss and interpret different responses that youth provided that were used as indications of the presence of specific psychopathy traits. This includes describing manifestations of each item from the Interpersonal, Affective, and Lifestyle factors from the PCL:YV.