If you are a real estate broker or agent, you may have had to pursue in collecting your commission or a dispute has arose of the procuring cause of the sale.  Here are the basic elements to be entitled to your commission: (1) the broker or salesperson must be licensed by the Department of Real Estate, (2) obligation to pay compensation must be made in writing, and (3) depending upon the terms of the agreement, the broker must have procured a ready, willing, and able buyer or tenant, prepared to buy or lease the property.

Of course, procuring cause is almost always the subject of litigation.  Usually this can be avoided with appropriate listing agreements.