Integrated circuits are manufactured through highly complex processes where tiny variations can introduce defects that are invisible at the design level. These defects may cause incorrect logic values, timing failures, or unstable behavior that only appears under certain conditions. Detecting such problems requires systematic methods that go far beyond simply checking whether a circuit appears to function correctly. Testing exists to expose hidden failures, control internal states, and observe circuit responses in a predictable and repeatable way. Testing relies on the ability to drive signals into a circuit and observe how the circuit reacts. When control or observation is limited, faults can remain hidden even when they exist in silicon. Physical defects are therefore represented using abstract fault models that describe how a circuit behaves when something is wrong. Static faults capture permanent logic failures, while dynamic faults represent timing-related behavior such as slow signal transitions caused by resistive defects. These abstractions make it possible to reason about failures without directly seeing the physical defect. As designs grow larger and include memory and ...