Recently, the transcription factor encoded by tumor suppressor gene p53 was shown to regulate the expression of microRNAs. The most significant induction by p53 was observed for the microRNAs miR-34a and miR-34b/c, which turned out to be direct p53 target genes. Ectopic miR-34 expression induces apoptosis, cell-cycle arrest or senescence. In many tumor types the promoters of the miR-34a and the miR-34b/c genes are subject to inactivation by CpG methylation. MiR-34a resides on 1p36 and is commonly deleted in neuroblastomas. Furthermore, the loss of miR-34 expression has been linked to resistance against apoptosis induced by p53 activating agents used in chemotherapy. In this review, the evidence for a role of miR-34a and miR-34b/c in the apoptotic response of normal and tumor cells is surveyed.