The internal restructuring of bone as a reaction to physiological load described by Wolff's law is related to both stress and strain in the bone. Bone is a rather brittle material, with nearly equal properties in tension and in compression: Elastic modulus E = 20 Gpa, tensile strength sigma tens = 200 Mpa (cortical bone). In biomechanics, it is very easy to assess strain using strain gauges. The theoretical and technical aspects specific to the use of strain gauges on bones are presented in this paper including how the measurements are taken and how the data is analyzed.