This Chapter addresses the problem of quality of service (QoS) provisioning in optical burst switching (OBS) networks. OBS is a photonic network technology aiming at efficient transport of IP traffic. The lack of optical memories, however,makes the operation in such networks quite complicated, especially if one wants to guarantee a certain level of service quality. Indeed quality demanding applications such as, for instance, real-time voice and video transmissions, need for additionalmechanisms so that to preserve them from low priority data traffic. In this context the burst blocking probability metric is perhaps of the highest importance in OBS networks. In this Chapter we present a general classification of QoS provisioning methods considered for OBS networks.We study several QoS scenarios that are based on the most referenced QoS mechanisms and we confront their performance in the same evaluation scenario consisting of a single isolated node. Among all the mechanisms analysed, the best overall performance is achieved with a burst preemptive mechanism. Since the preemptive mechanism produces the problem of resources overbooking in the network we address this issue as well.