Ataxia‐telangiectasia (A‐T) is characterized by neuronal degeneration, cancer, diabetes, immune deficiency, and increased sensitivity to ionizing radiation. A‐T is attributed to the deficiency of the protein kinase coded by the ATM (ataxia‐telangiectasia mutated) gene. ATM is a sensor of DNA double‐strand breaks (DSBs) and signals to cell cycle checkpoints and the DNA repair machinery. ATM phosphorylates numerous substrates and activates many cell‐signaling pathways. There has been considerable debate about whether a defective DNA damage response is causative of the neurological aspects of the disease. In proliferating cells, ATM is localized mainly in the nucleus; however, in postmitotic cells such as neurons, ATM is mostly cytoplasmic. Recent studies reveal an increasing number of roles for ATM in the cytoplasm, including activation by oxidative stress. ATM associates with organelles including mitochondria and peroxisomes, both sources of reactive oxygen species (ROS), which have been implicated in neurodegenerative diseases and aging. ATM is also associated with synaptic vesicles and has a role in regulating cellular homeostasis and autophagy. The cytoplasmic roles of ATM provide a new perspective on the neurodegenerative process in A‐T. This review will examine the expanding roles of ATM in cellular homeostasis and relate these functions to the complex A‐T phenotype.