This chapter highlights research pertaining to children of illicit drug users or substance users. It provides an in-depth examination of how proximal and distal environmental factors for children of illicit drug users are conceptualized and measured, as well as how the understanding and methodology can be improved. The maternal-child interaction is typically assessed using coded live observations or videotaped footage. A lack of parental response to family conflict also tends to be problematic in substance-using families and maladaptive family problem-solving and avoidance of familial conflict has been associated with reinforcement of substance use behavior. Typically conflict, communication, and cohesion are assessed together using self-report measures of the overall family environment, either from the child or parent's perspective. Evidence indicates that a myriad of environmental risk and protective factors across parenting behaviors, family environment, social context, and the neighborhood play a role in determining outcomes for children of illicit drug users.