It is generally accepted that PIWI proteins are predominately expressed in the germline but absent in somatic tissues. Their best-characterized role is to suppress transposon expression, which ensures genomic stability in the germline. However, increasing evidence has suggested that PIWI proteins are linked to the hallmarks of cancer defined by Weinberg and Hanahan, such as cell proliferation, anti-apoptosis, genomic instability, invasion and metastasis. This provides new possibilities for anticancer therapies through the targeting of PIWI proteins, which may have fewer side effects due to their potential classification as a CTA (cancer/testis antigen). Furthermore, PIWI has been proposed to act as a diagnostic and prognostic marker for many types of cancer, and even to differentiate early- and late-stage cancers. We herein summarize the latest progress in this exciting field, hoping to encourage new investigations of PIWIs in cancer biology that will help to develop new therapeutics for clinical application.