Deflectometers and extensometers (extensiometers) are used to indicate the deformation of material while it is subjected to stress. Extensometers indicate the deformation of material while it is subjected to tensile stress and deflectometers indicate the deformation of material while it is subjected to transverse or compressive stress..Extensometers are often based on strain gauge technology.  Laser extensometers measures strain by scanning across stripes painted on the sample. As the sample is stressed, the position of the stripes change, which affects the reflected laser. A tape extensometer uses a tape stretched between two reference points on large structures such as tunnel walls and bridge supports to detect changes in the distance between the two reference points. Borehole extensometers measure movement of soil and rock along the axis of a borehole. A rod extensometer is a borehole extensometer that uses rods to connect anchors in the hole with a reference head at the top of the hole. Material stress has two primary components: direct stress and shear stress.  Direct stress changes the volume of the material. Shear stress deforms the material without changing its volume. The modulus of elasticity (also known as Young's modulus or elastic modulus) is a measure of the stiffness of a given material. Fatigue testing determines the relationship between stress and the number of times it can be applied before causing failure. Testing machines are used for applying cyclically varying stresses and cover tension, compression, torsion and bending or a combination of these stresses.

Extensometers or extensiometers vary by measurement technology (electrical resistance or piezoresistive), arrangement (uniaxial, dual linear, strip gauges, diaphragm, tee rosette, rectangular rosette, delta rosette), number of gauges in gauge pattern, operating temperature, gage factor, nominal resistance, strain-sensitive material (e.g., A-alloy or platinum alloy) and termination options (ribbon leads, lead wires, or solder tabs). Some extensometers are suitable for cryogenic use, embeddable, or encapsulated. Others are waterproof or weldable. Specialty applications include crack detection, crack propagation, and temperature measurement.

ASTM International (formerly called the American Society for Testing and Materials (ASTM) maintains standards on extensometers or extensiometers, such as ASTM E83, standard practice for verification and classification of an extensometer system. The International Organization for Standardization (ISO) maintains ISO 9513 for metallic materials and calibration of extensometers used in uniaxial testing.