This report describes some basic principles of hydrology as they apply to Martin County, Florida. The major land features are described first, then the hydrologic cycle and surface-water and ground-water resource. Specific problems associated with population growth and the county 's development, such as saltwater intrusion and water use, are cited and sections have been included to deal with these problems. The report is intended to provide a background on the water resource of Martin County to those who do not have the time nor the need to delve deeply into the county 's hydrologic conditions or its water-management problems. It is freely illustrated and uses a minimum of text to explain the illustrations. General data on the water resources of the county are given, but few specific basic data are presented. (Woodard-USGS)