The growing use of small wireless devices is making mobile ad hoc networks indispensable as an effective solution for ubiquitous computing and internet of things. Moreover, these networks are considered as a communication solution for emergency or disaster situations. However, to be usable at this scale, mobile ad hoc networks need to be monitored if not totaly at least partially. Monitoring is a network management function that can be achieved inside the network without any external intervention. The purpose of monitoring in such networks is to collect information such as the functional states of the participating nodes and the operational states of the available routes and to report this information to all participating nodes or to some of them according to the network application. This paper outlines the challenges of monitoring over mobile ad hoc networks. We discuss monitoring issues and provide a comprehensive overview of existing monitoring approaches. We present a classification of these approaches based on mobile ad hoc network characteristics, their deployment issues and the way the network is organized. For each approach, we present a brief overview, then we describe the architecture and the proposed protocols; we discuss their advantages and limitations; and we compare the approaches based on common criteria such as message overhead, distribution, etc.