Anomalous gene expression is a major hallmark of cancer. Initially, it was demonstrated that altered activities of transcription factors were responsible for several cancer types. However, in the last decade, RNA arose as the key regulator of gene expression, whether in the form of protein-coding RNA or through several noncoding RNAs with a number of regulatory mechanisms that modulate transcription and translation in different ways. In this regard, it has been demonstrated that RNA processing, including alternative splicing, is systematically altered in cancer, showing the important role of RNA on tumorigenesis, cell growth, and cancer progression. On the other hand, the expression of noncoding RNAs such as miRNAs, lncRNAs, and circRNAs could be altered in the context of cancer, and they have been shown to promote tumorigenesis. Here, we present the general mechanisms by which RNA can regulate gene expression in the context of cancer, and we summarize the evidence that supports the central role of RNA in the onset and development of different cancer types. With all this information, we believe that new and promising tools could be developed in the near future, due to the application of RNA technology to cancer diagnosis and treatment.