Magnetic microscopy is a defect localization technique that has several advantages. It is nondestructive, noninvasive, and contactless. In many cases, it can be used even before component depackaging. This article describes the basic setup of a magnetic current imaging (MCI) microscope and explains how it reveals 3D current paths at the package and die level. It also presents application examples showing how MCI has helped failure analysts isolate a wide range of electrical defects, including shorts, resistive opens, and full opens.