A number of important ideas, developed during the past 100 years, form the framework of the present understanding of regional ground-water flow. The most important of these ideas are:

1. Differences in topographic elevation provide the principal driving force for regional flow.

2. Flow through confining layers forms an essential element of regional flow systems.

3. Chemical evolution within the flow systems can be used to understand the flow.

4. Moving ground water is an efficient transport mechanism for heat within the Earth.

5. We trace the evolution of these ideas in the United States and demonstrate their influence on the present-day understanding of flow systems with examples taken primarily from the American literature.