Generally, data collected by sensors must be sent to a sink node using multi-hop routing. Some applications can raise different requirements, constraints for the quality of data forwarding, for instance on end-to-end delay, jitter, packet loss, etc. Often, a set of constraints must be satisfied. Several proactive and reactive routing protocols have been proposed for WSNs. One of them is the standardized proactive RPL (Routing Protocol for Low-Power and Lossy Networks) designed essentially for many to one communication. For the data gathering a destination oriented directed acyclic graph (DODAG) is used which is mainly a tree directed to the sink. The computation of the tree-like DODAG is based on an Objective Function which can also be defined on QoS metrics but is usually based on one metric. This chapter deals with QoS routing in general and in WSNs using RPL in particular and proposes the analysis of the QoS constrained routes. We demonstrate that the set of QoS routes from sensors to the sink is not always a tree. It corresponds to a generalization of trees: to a hierarchy. The QoS aware hierarchies can be considered as special DODAGs. The routes are directed to the sink, and there is no useless cycle. The configuration of them and the data forwarding need some adjustments of the routing protocol which are also presented.