Teamwork in complex, dynamic, multi-agent domains mandates highly flexible coordination and communication. Simply fitting individual agents with precomputed coordination plans will not do, for their inflexibility can cause severe failures in teamwork, and their domain-specificity hinders reusability. Our central hypothesis is that the key to such flexibility and reusability is agent architectures with integrated teamwork capabilities. This fundamental shift in agent architectures is illustrated via an implemented candidate: STEAM. While STEAM is founded on the j&t intentions theory, practical operationalization has required it to integrate several key novel concepts: (i) team synchronization to establish joint intentions; (ii) constructs for monitoring joint intentions and repair; and (iii) decision-theoretic communication selectivity (to pragmatically extend the joint intentions theory). Applications in three different complex domains, with empirical results, are presented.