In this chapter we investigate a set of problems and challenges to introduce a monitoring mechanism in Wireless Sensor Networks (WSNs). The monitoring mechanism is a set of actions launched by the monitor sensor nodes to supervise the activities of their neighbor nodes called the monitored nodes. The aim of the monitoring mechanism is to detect the anomalies in the network related to the nodes' behavior (like non-cooperative nodes), to their availability (like end of battery life, sensing board problem, nodes mobility) and to the quality of the wireless communication link (like high collision rates, jamming attacks). Therefore, we can use the monitoring mechanism to efficiently administrate WSNs: detect and localize the origin of the communication problem, prevent the interruption of the communication, and get the normal communication back. The challenges to introduce the monitoring mechanism are mainly based on WSNs' characteristics: the resources are limited like the energy, the bandwidth, the processing unit capability, etc. In addition, we propose some solutions able to optimize and to adapt the monitoring mechanism to WSNs characteristics. We focus on IEEE 802.15.4 technology in both beacon-enabled and non-beacon-enabled modes to make the implementation of the monitoring mechanism possible. We study the case of the intentional non-cooperative or non-relaying nodes to evaluate the proposed solutions. Moreover, we analyze the cost of the monitoring mechanism in terms of energy consumption and the quality of detection by assessing the monitor's observation. The impact of nodes density, packets' size, network traffic load: saturated/unsaturated cases and distance between monitor and monitored nodes are taken into account in our evaluation.