Objective: To introduce item response theory (IRT) to health behavior researchers by contrasting it with classical test theory and providing an example of IRT in health behavior. Method: Demonstrate IRT by fitting the 2PL model to substance-use survey data from the Adolescent Health Risk Behavior questionnaire (n = 1343 adolescents). Results: An IRT 2PL model can produce viable substance use scores that differentiate different levels of substance use, resulting in improved precision and specificity at the respondent level. Conclusion: IRT is a viable option for health researchers who want to produce high-quality scores for unidimensional constructs. The results from our example--although not flawless--demonstrate the feasibility of IRT in health behavior research.