Following this examination of theories about what we can know and how we come to know it, the chapter applies the analysis to criminology that informs youth justice. The author concludes that youth justice is currently based on the modernist assumption that factual, reliable knowledge can be gained about human behavior and experience. Based on this knowledge, criminologists can identify the key risk factors for criminal behavior. Knowledge about risk factors in a particular youth's life can then be gained through various data-collection instruments. Based on this knowledge, methods can be designed to change risk factors and thus behavior. More recently, criminological theory and research informed by postmodern epistemology and ontology has emerged. Under this approach to knowledge, crime is not regarded as some naturally occurring phenomenon, but rather as a product of human efforts to find meaning, rewards, and well-being in the context of particular historical periods. This means that definitions of crime and human behaviors will vary from culture to culture and historical contexts. From this perspective, evidence to inform policy and practice is derived from in-depth, qualitative research that provides insight into why various interventions with particular individuals at particular times are either effective or ineffective in producing the desired behavioral change. This examination of what can be known about human behavior and how it is known is meant to encourage policymakers and practitioners to examine claims about "what works." 10 references