Catalase-peroxidases (KatGs) exhibit peroxidase and substantial catalase activities similar to monofunctional catalases. Crystal structures of four different KatGs reveal the presence of a peroxidase-conserved proximal and distal heme pocket together with features unique to KatG. To gain insight into their structure-function properties, many variants were produced and very similar results were obtained irrespective of the origin of the KatG mutated. This review focuses mainly on the electronic absorption and resonance Raman results together with the combined analysis of pre-steady and steady-state kinetics of various mutants involving both the peroxidase-conserved and the KatG-specific residues of recombinant KatG from the cyanobacterium Synechocystis. Marked differences in the structural role of conserved amino acids and hydrogen-bond networks in KatG with respect to the other plant peroxidases were found. Typically, the catalatic but not the peroxidatic activity was very sensitive to mutations that disrupted the KatG-typical extensive hydrogen-bonding network. Moreover, the integrity of this network is crucial for the formation of distinct protein radicals formed upon incubation of KatG with peroxides in the absence of one-electron donors. The correlation between the structural architecture and the bifunctional activity is discussed and compared with data obtained for KatGs from other organisms.