A major portion of the eukaryotic genome is occupied by DNA sequences; transcripts of these sequences do not code for proteins. This part of the eukaryotic genome is transcribed in a developmentally regulated manner or as a response to external stimuli to produce large numbers of long non-coding RNAs (lncRNAs). Genome-wide studies indicate the existence of more than 3,300 lncRNAs. Long non-coding RNAs are tentatively defined as molecules of ncRNAs that are more than two hundred nucleotides long. Due to the complexity and diversity of their sequences, progress in the field of lncRNAs has been very slow. Nonetheless, lncRNAs have emerged as key molecules involved in the control of transcriptional and posttranscriptional gene regulatory pathways. Although limited numbers of functional lncRNAs have been identified so far, the immense regulatory potential of lncRNAs is already evident, emphasizing that a genome-wide characterization of functional lncRNAs is needed. The fact that many lncRNAs are deregulated in various human cancers, together with their functional characteristics, implies their eminent role in carcinogenesis. In this review, we summarize novel classes of lncRNAs, describe their biological functions emphasizing their roles in tumor biology and translational oncology research.