All aquifers are heterogeneous to a certain degree. The spatial distribution of hydraulic conductivity K(x, y, z), or aquifer heterogeneity, significantly influences the groundwater flow movement and associated solute transport. Of particular importance in designing an in-situ remediation plan is a knowledge of low-K layers because they are less accessible to remedial agents and form a bottleneck in remediation. The characterization of aquifer heterogeneity is essential to the solution of many practical and scientific groundwater problems. This article reviews the field technique using the multilevel slug test (MLST), which determines a series of K estimates at depths of interest in a well by making use of a double-packer system. The K(z) obtained manifests the vertical variation of hydraulic conductivity in the vicinity of the test well, and the combination of K(z) from different wells gives rise to a three-dimensional description of K(x, y, z). The MLST response is rather sensitive to hydraulic conductivity variation; e.g., it is oscillatory for highly permeable conditions (K > 5 × 10^-4 m s^-1) and a nonoscillatory for K < 5 × 10^-4 m s^-1. In this article we discuss the instrumentation of the double-packer system, the implementation of the depth-specific slug test, the data analysis methods for a spectrum of response characteristics usually observed in the field, and field applications of the MLST.