A large proportion of urology involves the treatment of bladder outflow obstruction (BOO) in men. Yet BOO in men is very difficult to accurately quantify with the only regular non-invasive test being uroflowmetry. Pressure flow urodynamics may be considered the gold standard, but has the drawback of being an invasive test with the risks of haematuria and urinary tract infection. It can also be undignified with considerable rates of anxiety and embarrassment. The authors performed a comprehensive systematic review of the literature of non-invasive tests to assess their potential accuracy and suitability in the assessment of men with BOO. The minimum patient age was 18 and neurological disease or urethral stricture patients were excluded. Forty-two studies were reviewed, with a total of 4444 patients. The non-invasive tests reviewed were prostate volume / height, intravesical prostate protrusion (IPP), detrusor / bladder wall thickness measured on transabdominal ultrasound (DWT), ultrasound-estimated bladder weight (UEBW), doppler ultrasound, near-infrared spectroscopy (NIRS), uroflowmetry, penile cuff test (PCT) and external condom catheter method. Uroflowmetry has a varying maximum flow rate (QMAX) threshold in several studies, the majority cutoff value was 10ml/s. Detrusor wall thickness cutoff was 2mm in the majority of studies. They found detrusor wall thickness (median sensitivity 82%, specificity 92%), NIRS (median sensitivity 85%, specificity 87%), and penile cuff test (median sensitivity 88%, specificity 75%) showed the greatest diagnostic accuracy. Uroflowmetry with a maximum flow rate of <10ml/s was reported to have lower median sensitivity and specificity of 68% and 70%, respectively. Intravesical prostatic protrusion of >10mm was reported to have similar diagnostic accuracy, with median sensitivity of 68% and specificity of 75%. These results may suggest we over-rely on flow rate alone, when other non-invasive tests may be more accurate. Ultrasound measurements of the bladder wall and prostate seem promising in particular and could potentially be performed by trained urologists in an outpatient setting.