Hydraulic parameter identification is a crucial step in hydrogeological investigations. The book proposes a unique and generalized interpretation method for single and multiple pumping tests made in groundwater reservoirs with layered heterogeneity and with or without lateral anisotropy. This method eliminates the drawbacks of the numerous and frequently applied interpretation methods. The book also presents an introduction to inverse modeling, resulting in optimal parameter values with their joint confidence region and the corresponding residuals. Cross sections through this multidimensional region elucidate the relation between the shape of this region and some statistical parameters describing the reliability of the identified parameters. This method is demonstrated by means of five pumping or recharge tests.