Nowadays, wireless networks are used in most of the applications with radio technologies being used in all kinds of wireless networks. In all wireless links, the transmitted packets can be lost. How to identify the quality of a certain wireless link and achieve the best delivery performance over a certain wireless network is an open issue. In this thesis, the performance of wireless mesh network, wireless sensor network and cellular network have been investigated by the method of measurements, simulations and mathematical model. Several novel algorithms also have been proposed. We have proposed two mechanisms to estimate wireless links where better performance is achieved compared to the traditional algorithms. The achievements lead to a higher end-to-end throughput (faster information delivery) for the IEEE 802.11 networks. Meanwhile, an energy saving and interference reducing mechanism for wireless mesh and sensor networks has been proposed and validated which can dramatically save energy in those wireless networks. Impact of wireless link quality on heterogeneous cellular and ad hoc networks has been evaluated and the results could be used for Telecom operators for the cellular networks optimization. Test-beds for IEEE 802.11 and IEEE 802.15.4 have been built and the 3G cellular networks have been studied via simulations. We looked at the the impact of the wireless link quality to the MAC layer for the IEEE 802.11 and IEEE 802.15.4; the impact of the wireless link quality to the IEEE 802.11 network layer. To understand the link quality characteristics of these two radios, we carried out many measurement experiments with various indoor and outdoor scenarios. The measurement methods and results are presented in Chapter 2. To achieve accurate link quality estimation, we proposed two estimation methods for different scenarios in Chapter 3. In Chapter 4, we used accurate link quality information to determine the packet transmission power level in the IEEE 802.11 and IEEE 802.15.4's MAC layer and the measurement results show that the energy consumption and interference reduces. The accurate link quality estimation methods are further used in data rate adaptation in the IEEE 802.11's MAC layer (Chapter 5) and route selection in the IEEE 802.11's network layer (Chapter 6). The measurements using IEEE 802.11 devices show tremendous performance enhancement. Our algorithms and results can be useful for all kinds of protocol designs for IEEE 802.11 and IEEE 802.15.4 networks. Furthermore, we look at the end-to-end performance of the cellular multihop network which comprises of two different wireless networks, 3G UMTS/HSDPA network and IEEE 802.11 network in Chapter 7. When multiple TCP flows exist in the cellular multi-hop network, different flows may not share the cellular network's resources fairly due to the fact that each flow may have different topology and wireless link quality. We have proposed a weighted scheduling method which obviously alleviates this unfairness. In Chapter 8, a typical video streaming traffic, MPEG-4 is used to evaluate the end-to-end performance over the cellular multi-hop network. The results show that wireless link quality also has great impact on the end-to-end performance of MPEG-4 over the cellular multi-hop network. Our result can be used by the telecom operators to optimize the UMTS/HSDPA system for the cellular multi-hop scenario. We conclude our thesis and suggest the future research directions in Chapter 9.