The use of transdiagnostic mental health treatments in low resource settings has been proposed as a possible aid in scaling up mental health services. Modular, multi-problem transdiagnostic treatments can be used to treat a range of mental health problems and are designed to handle comorbidity. Two randomized controlled trials have been completed on one treatment – the Common Elements Treatment Approach, or CETA – delivered by lay counsellors in Iraq and Thailand. This paper utilizes data from two clinical trials to explore the delivery of CETA by lay providers, examining fidelity and flexibility of element use. Data were collected at every therapy session. Clients completed a short symptom assessment and providers described the clinical elements delivered during sessions. Analyses included descriptive statistics of delivery including selection and sequencing of treatment elements, and the variance in element dose, clustering at the counsellor level, using multi-level models. Results indicate that lay providers in low resource settings (with supervision) demonstrated fidelity to the recommended CETA elements, order and dose, and occasionally added in elements and flexed dosage based on client presentation (i.e. flexibility). This modular approach did not result in significantly longer treatment duration. Our analysis suggests that lay providers were able to learn decision-making processes of CETA based on client presentation and adjust treatment as needed with supervision. As modular multi-problem transdiagnostic treatments continue to be explored in low resource settings, research should continue to focus on ‘unpacking’ lay counsellor delivery of these interventions, decision-making processes, and the level of supervision required.