Stress urinary incontinence (SUI) is a debilitating disorder caused by malfunctioning of the urethral sphincter. Anatomical and histological properties of the sphincter, its innervation and supporting structures are explained in relation to the closing mechanism of the bladder outlet. Urethral sphincter function is discussed from the passive concept of urethral pressure transmission to the ’hammock theory’ and the role of the pubococcygeus muscles. SUI is caused by a combination of intrinsic sphincter deficiency and urethral hypermobility. The difficult interpretation of the parameters in urodynamic investigation to assess intrinsic sphincter deficiency (ISD) and/or urethral hypermobility is discussed. Electromyography (EMG) is valuable in the assessment of the overall urethral sphincter in relation to maneuvers (kinesiological EMG) and at the level of the muscle fiber (needle EMG). The diagnostic potential of circumferential surface EMG in the urethral sphincter is reviewed in relation to the EMG features of ISD.