Only about 2% of the human genome constitute protein-coding genes - nevertheless, medical research has focused mainly on this portion in recent decades. Since up to 70% of the human genome is transcribed into RNA, the genome contains much more non-coding information than coding, which is present in the cell as non-coding RNA (ncRNA). Many of these ncRNAs are highly expressed, specifically regulated and evolutionarily conserved, arguing in favor of their functional significance. MicroRNAs are the most well-known ncRNAs, but many other long ncRNAs exist. Differential ncRNA or microRNA expression patterns correlate with diagnosis or prognosis in many tumor entities and can thus serve as an extensive source of new biomarkers. The expression of the long ncRNA MALAT1 correlates with tumor development, progression or survival in lung, liver and breast cancer. Functionally active ncRNAs can provide novel insights into the mechanisms underlying tumor development. The large number of different, often as yet unidentified ncRNAs promises new stimuli for the diagnosis, prognosis and therapy of many diseases.