For decades, education researchers have been drawing attention to the importance of professional collaboration among teachers as a way to improve student achievement. But not all collaboration is equally effective. Andy Hargreaves and Michael O'Connor explain that the most successful and sustainable efforts are characterized by both solidity and solidarity, meaning that they draw on both expert knowledge and strong collegial relationships, creating what the authors call collaborative professionalism. A network of rural teachers in the Pacific Northwest who enhance curriculum across multiple schools and a lesson study program in Hong Kong are two examples of such successful collaborations.