Functional magnetic resonance imaging (fMRI) research seeks to inform psychiatric clinical practice. First, it seeks to improve scientific understanding of normative brain development, the perturbations that lead to mental illness, and the outcomes of interventions. Second, it seeks to develop clinical applications to guide individual-specific decisions (eg, diagnosis, treatment selection) and interventions (eg, brain-stimulation targeting). The maturation of fMRI has brought a critical prerequisite within reach: measurement of individual differences. We address critical gaps in measurement reliability, which, if not corrected, could jeopardize progress toward clinical utility.