An intronic hexanucleotide (GGGGCC) expansion in theC9orf72gene is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). In the decade following its discovery, much progress has been made in enhancing our understanding of how it precipitates disease. Both loss of function caused by reducedC9orf72transcript levels, and gain of function mechanisms, triggered by the production of repetitive sense and antisense RNA and dipeptide repeat proteins, are thought to contribute to the toxicity.Drosophilamodels, with their unrivaled genetic tractability and short lifespan, have played a key role in developing our understanding ofC9orf72-related FTD/ALS. There is noC9orf72homolog in fly, and although this precludes investigations into loss of function toxicity, it is useful for elucidating mechanisms underpinning gain of function toxicity. To date there are a range ofDrosophila C9orf72models, encompassing different aspects of gain of function toxicity. In addition to pure repeat transgenes, which produce both repeat RNA and dipeptide repeat proteins (DPRs), RNA only models and DPR models have been generated to unpick the individual contributions of RNA and each dipeptide repeat protein toC9orf72toxicity. In this review, we discuss howDrosophilamodels have shaped our understanding ofC9orf72gain of function toxicity, and address opportunities to utilize these models for further research.