Background: Small supernumerary marker chromosomes (sSMCs) are a clinical problem in prenatal and postnatal diagnostic cases. They include few well-defined clinical syndromes, like cat eye syndrome or Emanuel syndrome. However, they are also a unique model to do research on numerical as well as structural aberrations in the human karyotype. Aim of the study: Here we provide an update on the present knowledge on sSMC formation, shape, content and clinical consequences. Materials and methods: All relevant underlying data was taken from a free data-collection on sSMCs set up by Thomas Liehr (http://ssmc-tl.com/sSMC.html or http://markerchromosomes.wg.am/). Results: A comprehensive genotype-phenotype correlation for sSMCs is still not available and has been recently complicated by the detection of so-called discontinuous sSMCs, most likely based on formation by chromothripsis. Factors like presence of uniparental disomy of sSMC’s sister chromosomes, the latter also influenced by the shape of the sSMC, mosaicism, genetic content (they may be formed by material derived from one or more chromosomes), and if they are parentally derived or de novo may have influence on the phenotype of its carrier. Сonclusions: Here we summarize the present knowledge on sSMCs, and stress that for reasonable genetic counselling sSMCs must be comprehensively characterized for their potential parental and chromosomal origin, genetic content, potential influence of imprinting and mosaicism.