Distributed shared memory (DSM) systems provide a shared memory programming paradigm on top of a physically distributed network of computers. The DSM system removes the necessity for programmers to move data explicitly between processors. The principle challenge in the development of an efficient DSM system lies in reducing the amount of communication necessary to maintain coherence to an absolute minimum. This thesis presents Brazos, a DSM system for use in an environment of symmetric multiprocessor (SMP) personal computers that are networked together by industry-standard 100 Mbps FastEthernet. Brazos is distinguished by its use of application-level multithreading, selective multicast, adaptive runtime mechanisms, and a unique performance history mechanism. Through the detailed analysis of twelve scientific programs, we show that Brazos outperforms the current state-of-the-art software DSM system by an average of 83%, and outperforms a version of the same DSM system that has been altered to take advantage of SMP personal computers by an average of 32%. Our results indicate that networks of commodity personal computers using available PC networks and operating systems can perform comparably on a wide variety of scientific applications to more traditional networks of high-end engineering workstations.