MicroRNAs (miRs) are small noncoding RNAs that negatively regulate gene expression by binding to the three untranslated regions of their target mRNAs. Deregulations of miRs were shown to play pivotal roles in tumorigenesis and progression. Recent research efforts have been devoted to translating these basic discoveries into applications that could improve the therapeutic outcome of patients with cancer. MiR-34a is a highly conserved miR throughout many different species. In humans, there are three homologs (hsa-miR34a, hsa-miR-34b and hsa-miR-34c). Early studies have shown that miR-34a acts as a tumor-suppressor gene by targeting many oncogenes related to proliferation, apoptosis and invasion. In this review, we provide a complex overview of miR-34a, including regulating its expression, its known functions in cancer and future challenges as a potential therapeutic target in human cancers.