Although effective in treating a range of childhood psychiatric conditions, selective serotonin reuptake inhibitors (SSRI) have been implicated in the induction of an "activation syndrome" (characterized by symptoms of irritability, restlessness, emotional labiality, etc.) that may represent an intermediary state change that fosters suicidality. SSRI-induced activation syndrome is well-accepted by many clinicians and thought to be relatively common, particularly in children and teens. However, gaps exist in empirical data on phenomenology and tools for early detection. With this in mind, we report on a recently funded National Institutes of Health grant to develop a measure of behavioral activation to be completed in a clinical setting. We discuss the development of this measure--the Treatment-Emergent Activation and Suicidality Assessment Profile (TE-ASAP)--as well as psychometric results from a sample of youth with internalizing disorders who were at varying stages of SSRI treatment. Overall, psychometric data were quite promising, with the TE-ASAP demonstrating excellent reliability (i.e., internal consistency, inter-rater, short-term test-retest stability) and strong validity properties. Through further evaluation of the TE-ASAP in the context of a controlled multimodal trial in youth with obsessive-compulsive disorder, we hope to augment understanding of activation syndrome and, in turn, mitigate risks through early detection of this potentially life-threatening adverse effect.