Strain measurement in multi-axial compression tests on concrete using fiber optic sensors. The quantification of load-induced anisotropic damage under various multi-axial stress conditions requires the measurement of strain inside small specimens. Therefore, quasi-continuously measuring fiber optic sensors without a protective coating were preferred. This paper explains the challenges of strain measurement in multi-axial tests and the potential of quasi-continuous strain measurement with fiber optic sensors. The positioning of the measuring fibers by using a brass profile in the unreinforced test specimens is explained and the influence of this support frame on the concrete properties is described. The available measuring accuracies and special measuring requirements, especially regarding the transverse pressure-sensitive and non-alkali-resistant measuring fibers, are shown. A comparison with the frequently used strain measurement via brush deformation shows that despite all existing challenges the fiber optic sensors allow strain measurement in triaxial tests in a so far not achievable accuracy.