The name "Ultra-Compact Dwarf Galaxy" (UCD) was invented for a new type of astronomical object that has been discovered in cores of nearby galaxy clusters a decade ago. UCDs resemble globular clusters, but are up to 100 times more massive and slightly more extended. Their luminosities are comparable to those of nuclei of the most massive dwarf ellipticals or late-type spirals. Various formation scenarios have been brought forward to explain the origin and evolution of UCDs. Two of them seem to

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... e most promising: first, UCDs might be the remnant nuclei of galaxies that have been disrupted in the cluster environment. Second, UCDs might have formed from the agglomeration of many young, massive star clusters that were created during ancient merger events. In this contribution, I review the discovery history of UCDs, summarize their observational properties in the fundamental plane of stellar systems and discuss the implications for the different formation scenarios based on our current knowledge about these enigmatic objects.