The 2011 discovery of the pathogenic hexanucleotide repeat expansion (HRE) in C9orf72, the leading genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), marked a breakthrough in the effort to unravel the etiology of these conditions. Ten years later, clinicians are still working to integrate the implications of this discovery into the care of individuals with ALS and/or FTD. Consensus management guidelines for ALS do not comprehensively address the issue of genetic testing, and questions remain about whom to test, what counseling should be provided before and after testing, laboratory methods, and test interpretation. These challenges have contributed to inconsistent clinical practices and present barriers to patients wishing to access testing. This review summarizes the clinical impact of the discovery of the C9orf72 HRE, outlines ongoing challenges, and provides recommendations for C9orf72 testing, counseling, and research.