The Mountain Aquifer, which straddles the border between Israel and the Palestinian territories, is one of the most important water resources for both areas. It is estimated to supply between 600 and 700 million cubic meters of water per year, more than a third of annual water consumption in Israel. Israelis and Palestinians share the water that percolates into the Mountain Aquifer. Nearly all water consumed by Palestinians in the West Bank is pumped from this aquifer. While Israelis have access to other sources of water, the Mountain Aquifer remains an important drinking water resource for people living in the metropolitan areas of Jerusalem, Tel Aviv, and Be’er Sheva. Additionally, Israeli settlements in the West Bank utilize water from the Mountain Aquifer (Tagar, 2004). Generally, the water extracted from this source has been reliable and clean, especially when compared to the Coastal Aquifer in the Gaza Strip, which has been decimated by saltwater intrusion (Lein, 2000). Much concern remains, however, over the future health of the Mountain Aquifer. Because it is a karstic aquifer, the Mountain Aquifer has greater pollution vulnerability. Land use changes in Palestine, including increasing levels of irrigated agriculture, increased urbanization, and an increased volume of untreated sewage and solid waste have raised concerns in both Israel and Palestine about the Mountain Aquifer’s long term viability (Tagar, 2004; Harpaz, 2000). This paper seeks to address the issue of Mountain Aquifer pollution by asking which areas of the Mountain Aquifer are most at risk for pollution. A GIS model known as the PIMethod will be used to determine which places in the aquifer’s recharge zone are most vulnerable. The resulting map will be used as the basis for a brief discussion about potential development patterns, land use policies, and infrastructure plans that could minimize risk to the aquifer. The map will also be discussed as an important part of the proposed United Nations guidelines for the protection of trans-boundary watercourses. Finally, challenges relating to pollution prevention in the Mountain Aquifer will be compared to the challenges facing the Edwards Aquifer, a large karstic aquifer just north of San Antonio, Texas. First, however, an overview of karstic aquifers, the physical and political state of the Mountain Aquifer region, and other GIS vulnerability models is necessary.