This investigation reports the development of a measure of adolescent connectedness and estimates of its psychometric properties. A measure was developed to assess the ecological and developmental dimensions of adolescent connectedness, defined as adolescents' caring for and involvement in specific relationships and contexts within their social ecology. Exploratory and confirmatory factor analyses in studies one and two yielded theoretically consistent factor solutions. These models were cross-validated in studies three and four with three geographically and ethnically diverse adolescent samples totaling 1454 adolescents. The measure of adolescent connectedness demonstrated satisfactory inter-item and test-retest reliability and convergent validity across samples. Consistent with social control and problem-behavior therapy, two higher order factors emerged across all of these samples: conventional vs. unconventional connectedness. These two dimensions of connectedness were found to differentially explain substance use for delinquent and non-delinquent adolescents. Using this ecological assessment, adolescent connectedness appears to differ as a function of age, sex, and problem-behavior status; varies across relationships and contexts; reflects wither conventional or unconventional behaviors and attitudes; and can explain engagement in risk-taking behaviors. (Contains 62 references and 6 tables.) (Author)