As organizational entities, alliances are inherently complex to manage, and they are continually under threat from external contingencies that originate either in the partners’ own organization or in the external environment. The preceding chapters have discussed how these internal and external forces drive alliance development and the instruments alliance managers can use to develop win-win alliances. However, establishing and maintaining alliances also causes changes in partners’ organizations and environments, which in turn could force managers to modify their alliance design and management in an effort to maintain fi t between the alliance, the alliance partners and the environment. A misfi t could lead to deteriorating effi ciency or effectiveness and, in an extreme situation, could decrease the viability of the alliance or even decrease the viability of one or all alliance partners in the environment. Together, these relationships between the alliance, the alliance partner organizations and the environment constitute an alliance system. The dynamic interaction within an alliance system has been referred to as alliance co-evolution. This chapter starts by explaining what is meant by a co-evolutionary view on alliances, as well as why, when and how alliance co-evolution occurs. The third section uses the alliance development framework to present managerial guidelines, and the following two sections conclude with a summary and a case illustration.