With the emergence of various applications that have different Quality of Service (QoS) requirements, the capability of a network to support QoS becomes more and more important and necessary. This dissertation explores QoS in Cognitive Packet Networks (CPN) by using adaptive routing, flow and congestion control. We present a detailed description and analysis of our proposed routing algorithms based on single and multiple QoS constraints. An online estimation of packet loss rate over a path is introduced. We implement and evaluate the adaptive routing scheme in an experimental CPN test-bed. Our experiments support our claims that the users can achieve their desired best-effort QoS through this routing scheme. We also propose a QoS-based flow and congestion control scheme that is built in the transport layer and specially designed to work with CPN to support users' QoS while remaining friendly to TCP. Theoretical models and experimental analysis are presented. Finally we experimentally demonstrate that the proposed flow and congestion control scheme can effectively control the input flows, react to the congestion and work with our proposed adaptive routing scheme to achieve users' QoS.