The aim of this book is to reveal to a large spectrum of audience including biologists and physicians the extent of the microRNAs revolution in the cancer society. Alterations in miRNA genes play a critical role in the pathophysiology of many, perhaps all, human cancer: cancer initiation and progression can involve microRNAs (miRNAs) - small non-coding RNAs that can regulate gene expression. At the present time, the main mechanism of microRNAs alteration in cancer cells seems to be represented by aberrant gene expression, characterized by abnormal levels of expression for mature and/or precursor miRNA sequences in comparison with the corresponding normal tissues. Loss or amplification of miRNA genes has been reported in a variety of cancers and altered patterns of miRNA expression may affect cell cycle and survival programs. Germline and somatic mutations in miRNAs or polymorphisms in the mRNAs targeted by miRNAs may also contribute to cancer predisposition and progression. The causes of the widespread differential expression of miRNA genes between malignant and normal cells can be explained by the genomic location of these genes in cancer-associated genomic regions, by epigenetic mechanisms as well as by alterations of members of the processing machinery. MicroRNAs expression profiling has been exploited to identify miRNAs that are potentially involved in the pathogenesis of human cancers. MicroRNAs profiling achieved by various methods has allowed the identification of signatures associated with diagnosis, staging, progression, prognosis and response to treatment of human tumors.