Small stones in the kidney may be left untreated and simply observed to see whether they grow over time.  If these small stones drop into the ureter they may pass spontaneously.  The speed at which they are passed down the pipe can be increased by using tablets (Tamsulosin), which relaxes the ureter.  

In patients with larger stones (greater than 4mm), or with stones that are not moving down the ureter satisfactorily, treatment may be recommended as the chances of passing the stone without treatment falls.  Likewise, stones in the kidney that are likely not to pass should they fall into the ureter pipe may also be treated.