How do you know whether or not you need a written contract?
First, depending on where you live, laws require that certain types of contracts be in writing in order to be valid. For example, if a contract can not be completed within one year, or a contract to buy or sell real estate may need to be in writing to be enforceable. Make an appointment with a business lawyer in your state to find out what your state requires.
Secondly, getting a written contract that outlines the agreed upon terms and conditions may help if there is a dispute later on down the road. A written contract defines the obligations of the parties. This can prevent a party from attempting to back out of the contract or saying that they agreed to different terms and conditions than you think.