MicroRNAs (miRNAs) are a class of endogenous non-coding small RNAs that are evolutionarily conserved and widely distributed among species. Their major function is to negatively regulate mRNA target genes, and miRNA expression has been found to be deregulated in all human cancers, where miRNAs play critical roles in tumorigenesis, functioning either as tumor suppressors or as oncogenes. This review provides a current overview of the connection between miRNAs and cancer by covering the recent advances in miRNA involvement in human cancer including initiation, growth, invasion, and metastasis. We will also highlight the literature where application of miRNAs has created the foundation for the development of potential future miRNA therapy.