SWI/SNF chromatin remodeling complexes are pleomorphic multisubunit cellular machines that utilize the energy of ATP hydrolysis to modulate chromatin structure. The complexes interact with transcription factors at promoters and enhancers to modulate gene expression and contribute to lineage specification, differentiation, and development. Initial clues to a role in tumor suppression for SWI/SNF complexes came over a decade ago when the gene encoding the SMARCB1/SNF5 core subunit was found specifically inactivated in nearly all pediatric rhabdoid tumors. In the last three years, cancer-genome sequencing efforts have revealed an unexpectedly high mutation rate of SWI/SNF subunit genes, which are collectively mutated in 20% of all human cancers and approach the frequency of p53 mutations. Here, we provide a background on these newly recognized tumor suppressor complexes, discuss mechanisms implicated in the tumor suppressor activity, and highlight findings that may lead to potential therapeutic targets for SWI/SNF-mutant cancers.