In teaching leadership development we have developed and revised a model of teamwork and collaboration, which has yielded innovative and positive results. Our study draws on insights from more than 90 project teams, gathered over twelve years of a mid-career executive education program designed specifically to teach collaborative leadership. The teams work on a strategic dilemma with a business association or community organization, highlighting the civic engagement aspect of collaborative leadership. Teams devise their own operating procedures, refine (not simply manage) the project, create working relationships with multiple stakeholders, and present a deliverable within the nine-month span of the program. The team experience emphasizes complexity and ongoing reflection. We identify seven concrete leadership skills that emerging leaders developed: coping with ambiguity, working cross-functionally beyond their usual expertise, knowing when to get outside help, understanding different stakeholders, working effectively across dimensions of diversity, dividing labor in a leaderless team, and handing off a project that advances but does not close a complex problem. Rather than implementing flawlessly on a project with defined metrics--which propels potential leaders partway up the ladder--participants shift toward the greater leadership challenges of defining scale and significance, changing course after listening carefully, and learning from obstacles. They emerge from the crucible of teamwork with leadership skills for everyday work and the future.