Ambulatory assessment methods have made it possible to study psychological phenomena in real-time, with translational potential for psychotherapy process research. This article uses case example data to demonstrate applications of ambulatory assessment to measuring emotion regulation, a process with relevance across diagnoses and treatment modalities that may be particularly important to measure in situ. Two methods are reviewed: Ecological Momentary Assessment (EMA), which enables self-reported momentary assessments as people go about their days, and the Electronically Activated Recorder (EAR), an unobtrusive naturalistic observation methodology that collects short audio recordings from participants' moment-to-moment environments, capturing an acoustic diary of their social interactions, daily behaviors, and natural daily language use. Using case example data from research applying EMA and EAR methods in the context of adolescent self-injurious thoughts and behaviors, we illustrate how EMA can be used to measure emotion regulation over time and across contexts, and how EAR can assess the behaviors and social-environmental factors that interact with emotion regulation in clinically important ways. We suggest applications of this measurement approach for investigations of clients' emotional change over the course of psychotherapy, as well as potential clinical applications of these methods. Deanna M. Kaplan https://orcid.org/0000-0002-9300-3029 Nicole R. Nugent https://orcid.org/0000-0001-8756-5618 Deanna M. Kaplan served as lead for conceptualization, writingoriginal draft, and writingreview & editing, contributed equally to formal analysis and methodology, and served in a supporting role for project administration and visualization. Christopher D. Hughes served as lead for visualization, contributed equally to formal analysis and methodology, and served in a supporting role for writingoriginal draft and writingreview & editing. Heather T. Schatten served in a supporting role for visualization and writingreview & editing. Matthias R. Mehl served as lead for data curation, contributed equally to methodology, and served in a supporting role for funding acquisition, investigation, project administration, visualization, writingoriginal draft, and writingreview & editing. Michael F. Armey contributed equally to methodology and served in a supporting role for funding acquisition, investigation, project administration, visualization, writingoriginal draft, and writingreview & editing. Nicole R. Nugent served as lead for funding acquisition, investigation, methodology, project administration, and resources and served in a supporting role for conceptualization, data curation, formal analysis, visualization, writingoriginal draft, and writingreview & editing.