Educators, researchers, politicians, and the media have committed considerable time, attention, and effort to chronically low-performing schools. Although many have chided the schools for their unacceptable performance or designed strategies to improve them, few have sought to understand how these schools became chronically low-performing in the first place. Better knowledge of how schools decline into chronic low-performance will allow interested stakeholders to potentially prevent and turnaround chronically low-performing schools. However, before understanding the factors and processes associated with or contributing to school decline, researchers must first properly define and identify the process. This paper offers four operational definitions of school decline, applies those definitions to a population of schools, and evaluates the utility of each definition.