The ataxia-telangiectasia mutated (ATM) protein kinase plays a critical role in activating the cellular response to DNA double-strand breaks and promoting homology-directed repair. ATM is frequently mutated in cancer, contributing to an accumulation of DNA damage that drives genomic instability. To exploit cancer cells' inherent vulnerability to DNA damage, various small molecule inhibitors have been developed that target ATM. ATM inhibitors have shown great versatility in preclinical studies and increasing use in the clinic. Here, we review the development of ATM inhibitors and their role in cancer therapy. We describe their limitations and the advances that have led to increases in both the number and diversity of active clinical trials targeting ATM. We also discuss ATM's role in personalized medicine and the current challenges to more widespread use of ATM inhibitors in the clinic.