Urinary frequency and nocturia (waking up at night to void) can occur with prostate obstruction or with an overactive bladder. The treatment for each cause is very different and the best way to usually tell the difference is by having a bladder ultrasound after voiding to see how much urine is left over. If an abnormally high amount of urine is left over, then prostate obstruction is the most likely cause in men although other causes, such as scar tissue in the urethra are also possible. If the amount left in the bladder is very small then there is not likely any obstruction, and overactive bladder is more likely. The causes of overactive bladder are variable and should be identified in each case if possible, by your urologist and there are numerous possible treatments as well.