The objective of unsteady flow tests is to determine the aquifer parameter estimations from time-drawdown field measurements in the main and/or observation wells by processing them through physically based and simplified mathematical models. These models help to infer aquifer, well, or pump properties and finally to make the future predictions of discharges, drawdowns, or distances between wells for the groundwater resources planning, design, operation, and management. This chapter concentrates on the development of physically based models without entering the mathematical morass. The geological and physical procedures are presented as necessary tools for analyzing complex natural aquifers. For those readers interested in detailed mathematical involvements, consult reference books by Muskat (1937), Hantush (1964), Bear (1981), and Marino and Luthin (1982).