Considerable progress has been achieved in the experimental investigation and theoretical understanding of isovector valence shell structures in heavy nuclei that are frequently called mixed-symmetry states. This article attempts a review of the experimental aspects and data on mixed-symmetry states of vibrational and weakly deformed transitional nuclei. After a brief introduction the main experimental tools are described that have lately been used for the investigation of mixed-symmetry states in vibrational and transitional nuclei. The main body reviews the available data where the mixed-symmetry assignments have been made on the basis of large magnetic dipole transition rates.