Agricultural advisory services are important to improve farm performances and the farmers' ability to innovate. However, their effectiveness is regularly called into question. In this chapter, we discuss the evolution of agricultural advisory services and show how the mechanisms and methods used to provide advice have changed over time. There exist several different approaches to providing advice, based on different principles, which include decision support, problem resolution, capacity building for farmer empowerment, and support for individual or collective projects. The choice of an approach depends not only on the nature of the problem to be addressed and the solutions to be implemented, but also on the capacities of the advisers, the objectives set by advisory organizations, and the mechanisms for the governance and funding of agricultural advisory services.