Recent advances in micro-electro-mechanical systems (MEMS) technology, wireless communications, and digital electronics have enabled the development of low-cost, low-power wireless sensor nodes that are small in size and communicate untethered over short distances. Networking together hundreds or thousands of wireless sensor nodes allows users to accurately monitor a remote environment by intelligently combining data from the individual nodes. Such networks require robust wireless communication protocols that are energy efficient, data transmission efficient and support long network lifetime. In this Ph.D. dissertation, we analyzed and implemented the Low-Energy Adaptive Clustering Hierarchy (LEACH) protocol, a leading protocol architecture for wireless sensor networks. LEACH combines the ideas of energy-efficient cluster-based routing and media access together with application-specific data aggregation to achieve a desired performance. In an effort to improve the power consumption and network lifetime, we proposed Dynamic LEACH, or D-LEACH, a data-adaptive hierarchical protocol based on LEACH. The main idea of D-LEACH is to dynamically change the likelihood for each node to send data to the base station, based on the similarity of the data within each node cluster. We implemented D-LEACH and LEACH on the TinyOS platform on hardware motes, ran experiments to analyze D-LEACH performance, and compared against other major protocols. The analysis shows that in many cases D-LEACH achieves a superior performance when compared to LEACH and XMesh protocols, in terms of power consumption, total data received, and network lifetime. In the second part of this dissertation, we proposed and implemented a vital sign monitoring system based on wireless sensor network hardware. Our system combines wireless sensor network, vital sign monitoring technologies to monitor users' vital signs and notify relatives and medical personnel of users' status during life threatening situations. The use of wireless sensor network technologies makes it possible to deploy our system in all types of homes and facilities.