The Guidebook for evaluating fisheries co-management effectiveness offers a process and method to evaluate the performance of a fisheries co-management system and its plan in order to enhance its effectiveness in delivering benefits and in contributing to environmental, social and economic sustainability and good governance. It is to be used to evaluate the effectiveness of an existing fisheries co-management system operating at a fishery, community or sector level, or in a spatially defined area. It presents a flexible approach that can be used in many types of fisheries co-management systems with different contexts and characteristics. The primary audience for the Guidebook is those who commission an evaluation and those who carry out the evaluation. Both of these audiences may include, but not be limited to, government, fishers and other resource users, donors, non-governmental organizations, academic institutions, and research centres and institutes. The fisheries co-management effectiveness evaluation in the Guidebook is recommended to be undertaken in two separate but complementary parts: evaluating the implementation design and process of the fisheries co-management system, and evaluating the achievements of the goals and objectives that are stated in the fisheries co-management plan. The process for conducting a fisheries co-management effectiveness evaluation is undertaken in three steps, namely planning, compilation of information, and evaluation. For each step, a set of tasks and activities is presented. A variety of recommended indicators, used in measuring effectiveness, that reflect a diversity of fisheries co-management system good practices, and fisheries co-management plan goals and objectives, are provided. The evaluation of co-management effectiveness is linked to routine operational monitoring and to adaptive management, a cyclical process of systematically “learning by doing”. The results of the evaluation are used by the co-managers to better understand why goals and objectives and expected impact have or have not been achieved and to adapt comanagement design, processes and actions. Evaluation will also improve knowledge of fisheries co-management more generally