Abstract
DNA within cells is either present in the form of long strands as in eukaryotes or circular shapes in Yeast plasmids, mitochondrial DNA, and double minutes in tumor cells. Apart from them, ribosomal or telomeric DNA has been found to produce specialized forms of extrachromosomal circular DNA (eccDNA). eccDNA was discovered in both normal and cancer cells in recent times, indicating a much more significant role. The eccDNA has been found to promote tumor proliferation, survival, and aggressiveness in almost half of all cancers by increasing oncogene copy numbers. This chapter will discuss the biogenesis and function of eccDNA and how it promotes tumor adaption under changing microtumour environmental conditions, as in the case of drugs.