A Mobile Ad-hoc NETwork (MANET) is composed of a group of mobile wireless nodes that form a network independently of any centralized administration, while forwarding packets to each other in a multi-hop manner. Such networks are characterized by dense node deployment, unreliable sensor node, frequent topology change, limited power and memory constraints. Due to the bandwidth constraint and dynamic topology of MANET, it is especially significant to be able to provide Quality of Service (QoS). However, the characteristics of these networks makes QoS support a very challenging task. In this paper, the authors present the various QoS models and also focuses on the various challenges involved in providing QoS in MANET.