Recent years have witnessed an extensive proliferation of wireless technology in every domain of day-to-day life. Examples include mobile phones, broadband communication, wireless LAN, wireless enabled PDAs, cordless phones, garage-door openers and the list continues. Advancements in radio technology, antenna technology, low power computational digital signal processing (DSP) and micro-electro-mechanical systems (MEMS) are instrumental in reducing the size and cost of wireless devices. A wireless network consists of wireless devices forming an infrastructure-based or a peer-to-peer network. A network can be a single-hop or multihop network. Single-hop networks are already in existence and have been substantially investigated. This dissertation thus focuses on multihop wrireless networks, where the intermediate wireless devices also act as routers. Depending on their functionality, multihop wireless networks can be categorized into ad hoc, mesh and sensor networks. A mobile ad hoc network (MANET) aims at provding a mobile network with connectivity similar to a wired network without the need for any infrastructure support. A wireless mesh network (WMN) typically extends the infrastructure based single hop wireless network and has become a new paradigm for providing last mile broadband access. A wireless sensor network (WSN) is similar to an ad hoc network, providing a cheap alternative to monitoring applications. Each of these multihop wireless networks has their own set of challenges with respect to operation and implementation. The first part of this dissertation focuses on developing on-demand medium access control (MAC) protocols for multiple beam smart antennas (MBSAs) in ad hoc and mesh environments. MBSA has the unique capability of simultaneously initiating packet transmissions or receptions in multiple beams. Thus, compared to traditional omnidirectional antennas, MBSA can better utilize the spatial bandwidth, thereby increasing the capacity of wireless networks. We have performed both simulation and analytical studies to evaluate the proposed protocols for MBSA in ad hoc environments. To the best of our knowledge, this is the first attempt to analyze and develop on-demand protocols for multiple beam smart antennas. We have also proposed a cost-effective mesh network architecture employing heterogeneous antenna technologies and hybrid MAC protocol. The second part of this dissertation focuses on designing energy-efficient and reliable medium access mechanisms for wireless sensor networks. Sensor motes are battery-operated, hence protocols designed for them have to be innately energy-efficient. Also, depending on the application, reliability and latency might be important parameters. Taking into account all these design considerations, we have proposed dual-radio architecture. A low-energy wakeup radio is used to transmit and receive wakeup tones, while another transceiver is used for data communication. We have demonstrated the superior performance of our protocol using extensive simulation and analytical studies. We have also proposed a wireless sensor network testbed for quantifying reliability of wireless channels. The setup can be used to quantify reliability of wireless channels in terms of packet error rate, received signal strength and overall latency of the system. On the basis of our studies, we have provided deployment guidelines and medium access strategies for wireless sensor networks.