Individuals with co-occurring mental health and substance use disorders (i.e., concurrent disorders) have complex healthcare needs, which can be challenging for nurses to manage. Providing optimal care for this subpopulation requires nurses to develop high-level competencies despite limited resources at their disposal and the isolated settings in which many of them work. The Extension for Healthcare Community Outcomes (ECHO®) is a promising collaborative learning and capacity building model that uses videoconference technology to support and train healthcare professionals in the management of complex and chronic health conditions. The aim of this study was to explore the experiences and perceptions of nurses participating in a Canadian ECHO programme on concurrent disorders about the competencies they developed and used in their clinical practice, and which factors have influenced this process.