Law enforcement agencies are often first responders for people experiencing mental health or substance use crises because most communities have few other options. Many communities are exploring alternative crisis-response strategies to better connect people in crisis to the services they need. One alternative model in Denver, Colorado, called the Co-Responder program, pairs police with clinical professionals. Denver has committed to a robust evaluation of the Co-Responder program implementation, outcomes, and costs. This brief outlines the methods and key findings from the first year of a two-year mixed-methods evaluation of the Co-Responder program, including recommendations and next steps as the evaluation enters its second year. It focuses on understanding program implementation, including successes and challenges, and on informing practitioners and policymakers who are seeking to implement and improve similar programs.