In this lecture, we discuss the assumptions and applications of the Theis equation for groundwater analysis. Theis made several assumptions in deriving the equation, including that the aquifer is homogeneous, isotropic, and infinite, and that the transmissibility remains constant and water is discharged instantaneously. In practice, these assumptions are rarely met due to natural variations in aquifer properties and construction factors. The lecture also addresses the application of the Theis equation to unconfined aquifers, where transmissibility decreases with dewatering, leading to greater drawdowns compared to confined aquifers. To account for this, Jacob’s adjustment equation can be used to estimate drawdown as if the aquifer were confined.