The hardening process by heat treatment can alter the material properties of an alloy in a confined area, commonly the near-surface domain. The surface-/case-hardened component becomes scratch and wear resistant at the surface and the unhardened core remains ductile. In this chapter, nondestructive methods for determining the hardening depth are examined. Hardened components with a smooth decrease in hardness from the hardened area to the core are of particular interest. The determination of the surface/case depth for soft gradients of the material properties is a novel approach in nondestructive testing. The benefits, limits, and the industrial applicability of the testing procedures are considered. The different state-of-the-art NDE methods to measure the hardening depth are categorized by the physical mechanism used for the evaluation. The first group of methods consists of electromagnetic methods, the second of acoustic methods, and the third group encompasses thermal methods. The nondestructive methods for the determination of the hardness depth are measuring different material properties related to the hardness, not the hardness itself. The correlations and dependencies between material properties are a complex issue with individual aspects. A broad transfer of NDE data and the application of machine learning-based algorithms arises the opportunity to improve the NDE methods significantly (NDE 4.0). The testing options can be expanded and dependencies between material properties can be revealed. With the NDT methods presented, automated process control, comprehensive quality control, and a better understanding of material behavior can be achieved.