Protein-coding genes only account for about 2% of the total human genome. Recent technologies (e.g., tiling arrays and RNA-sequencing) have revealed that about 70% of the human genome is transcribed into noncoding RNAs. Long noncoding RNAs (lncRNAs), which comprise of a class of RNA molecules greater than 200 nucleotides in length, have been emerging as critical players in a broad range of cellular processes. More recently, a dozen of human lncRNAs have been implicated in driving cancer development through their oncogenic or tumor suppressive functions. This review aims to broadly define the functions and mechanisms of lncRNAs in the context of cancer, as well as discuss their potential future applications in the clinic.