Bladder outflow obstruction (BOO) may occur due to several underlying causes in both men and women. It is not possible to diagnose bladder outlet obstruction on a history alone. It can be suspected based on the use of a flow rate but can only be diagnosed using pressure flow urodynamics. In this chapter, we discuss the aetiology, pathophysiology, and investigation of BOO. We emphasize the importance of a complete history, examination, and investigations with investigations such as flow rate and voiding cystometry, in addition to standard tests including urinalysis and a bladder diary. The management of the underlying disorder responsible for the BOO symptoms is discussed in the relevant chapters separately.