Objective: A complaint of insomnia may have many causes. A brief tool examining contributing factors may be useful for nonsleep specialists. This study describes the development of the Insomnia Symptoms Assessment (ISA) for examining insomnia complaints. Method: ISA questions were designed to identify symptoms that may represent 1 of 8 possible factors contributing to insomnia symptoms, including delayed sleep phase syndrome (DSPS), shift work sleep disorder (SWSD), obstructive sleep apnea (OSA), mental health, chronic pain, restless leg syndrome (RLS), poor sleep hygiene, and psychophysiological insomnia (PI). The ISA was completed by 346 new patients. Patients met with a sleep specialist who determined primary and secondary diagnoses. Results: Mean age was 45 (18–85) years and 51% were male. Exploratory factor analysis (n = 217) and confirmatory factor analysis (n = 129) supported 5 factors with good internal consistency (Cronbach’s alpha), including RLS (.72), OSA (.60), SWSD (.67), DSPS (.64), and PI (.80). Thirty percent had 1 sleep diagnosis with a mean of 2.2 diagnoses per patient. No diagnosis was entered for 1.2% of patients. The receiver operating characteristics were examined and the area under the curves calculated as an indication of convergent validity for the primary diagnosis (N = 346) were .97 for SWSD, .78 for OSA, .67 for DSPS, .54 for PI, and .80 for RLS. Conclusion: The ISA demonstrated good internal consistency and corresponds well to expert diagnoses. Next steps include setting sensitivity/specificity cutoffs to suggest initial treatment recommendations for use in other settings.