Protein citrullination (PC) is a posttranslational modification (PTM) that converts a peptidyl arginine into a peptidyl citrulline. Aberrant PC is a hallmark of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease, Parkinson's disease, prion disease, and multiple sclerosis. Common among these diseases is a dramatic increase of PC in reactive astrocytes. Some citrullinated proteins have been identified. The most prominent are astrocytic cytoskeletal proteins such as GFAP and vimentin, and myelin protein MBP. Recent investigation in ALS has revealed new changes, including a decreased PC in neurons and an association of PC with myelin protein aggregates. These findings suggest that PC contributes to protein aggregation, neuronal dysfunction, neuroinflammation, and axonal degeneration. However, how PC impact neurodegeneration remains to be understood. Further studies are needed to understand a range of questions, from how PC modulates individual protein functions to its impact on diseases. Because of the PC's robust changes in neurodegenerative diseases, there are also prospects that this PTM may be harnessed as biomarkers, and modulation of this PTM may be an avenue for therapy. In this review, we summarize the current understanding of PC in ALS and other neurodegenerative diseases, the investigative methods for PC, and PC's potential as a biomarker and a therapeutic target.