The load-bearing capacity of concrete-to-concrete interfaces is a decisive factor in the strengthening and repair of reinforced concrete structures. To represent the transfer of forces across joints in the normal (tensile) and tangential (shear) directions, various approaches have been developed, typically based on the Mohr-Coulomb failure criterion. The model parameters can be derived from different experimental setups. This paper provides an overview and comparison of several studies, identifying the key factors that influence the mechanical behavior of the interface. The studies were selected to reflect material combinations representative of those used in practical structural strengthening applications. In particular, the effects of surface roughness, concrete age, and the compressive strength of both the existing and the supplementary concrete under tangential and normal loading are examined. In addition, the influence of different experimental setups is compared. The results show that only limited correlations can be established between studies—partly due to differences in the methods used to characterize concrete properties and partly due to variations in experimental configurations. Different experimental setups sometimes yield significantly divergent results. Based on these findings, a recommendation is derived regarding the key aspects to consider when conducting tests to characterize the properties of concrete-to-concrete interfaces.