||Rapid development in telecommunication technologies and ever-growing network-user demands have made network congestion a critical problem in today's Internet. Congestion not only leads to significant network-centric performance degradation such as low link utilization, but also damages the user-centric Quality-of-Service (QoS). In addition, the increasingly heterogeneous network environments plus varying and unpredictable traffic conditions suggests that any mechanism without adaptability hardly be able to deliver QoS requirements. This thesis considers the issue of congestion control and two QoS issues brought up by congestion in various network infrastructures including wired networks and wireless LANs. The two QoS issues are: (i) bandwidth assurance in the Assured Forwarding (AF) service in Differentiated Services (DiffServ) wired networks; and (ii) application level fairness between downlink and uplink flows in an infrastructure WLAN using the DCF mode. This thesis explores using a control theoretic approach to analyze and design mechanisms for these problems based only on the local knowledge. First we describe a generic controller structure and discuss its features. Then, we use it to design a stable queue-based adaptive AQM mechanism for the current Internet. We aim to alleviate two tradeoffs: (i) between fast transient response and stability, and (ii) between fast transient response and small steady state error. Stability analysis of the closed-loop nonlinear system is conducted, providing guidelines for designing the algorithm and selecting control parameters. The current Internet is a best-effort network and most AQM schemes focus on providing QoS assurance from a network-centric standpoint. The DiffServ approach has been proposed as a scalable mechanism in order to provide QoS. However, the AF service in this approach fails providing bandwidth assurance under certain conditions. In this thesis, we instantiate the generic controller structure in the AF service framework and analyze the features of the existing ingress-based mechanisms for improving bandwidth assurance among AF flows. Based on the analysis, we propose a simple but robust controller for this problem. Extensive simulations are carried out to validate the analysis and demonstrate that the proposed mechanism outperforms the mechanisms proposed in the literature over a wide range of network dynamics. Congestion control and fairness issues are not only prominent for the Internet infrastructure when wired backbones and centralized control entities are available, but are also essential for various applications that use wireless technologies for flexible data communication. Therefore, this dissertation extends the research methodologies and technologies for Internet congestion control and QoS problems to wireless LANs. We explore the feasibility of combining AQM schemes with the underlying MAC protocol design in WLANs to provide fairness for traffic flows traversing between wired and wireless domains.