Introduction -- Inheritance of small supernumerary marker chromosomes -- Formation of small supernumerary marker chromosomes -- Small supernumerary marker chromosomes in genetic diagnostics and counseling -- Small supernumerary marker chromosomes known to be correlated with specific syndromes -- Centric small supernumerary marker chromosomes -- Neocentric small supernumerary marker chromosomes by chromosome -- Multiple small supernumerary marker chromosomes -- Small supernumerary marker chromosomes additionally to other chromosomal rearrangements -- Complex small supernumerary marker chromosomes -- Small supernumerary marker chromosomes and tumors -- Appendix: Patient organizations in connection with small supernumerary marker chromosomes -- Glossary