This chapter focuses on the Balloon Expulsion Test (BET), a simple and cost-effective physiological assessment used to evaluate defecatory function in patients with suspected defecation disorders, a subtype of functional constipation. BET involves inserting a balloon into the rectum, inflating it with water or air, and then attempting to expel the balloon to simulate defecation. It is particularly useful for identifying patients who may benefit from biofeedback therapy, which has shown greater efficacy than laxatives or dietary fiber in treating defecation disorders. Although there is no standardized protocol for BET, failure to expel the balloon within 1–2 minutes is generally considered abnormal. This chapter discusses the diagnostic utility of BET in comparison with anorectal manometry and defecography, highlighting its high specificity but limited sensitivity. While BET alone cannot confirm or exclude a diagnosis, it remains a valuable tool when interpreted alongside clinical symptoms and other physiological tests.