While in some cases a handshake agreement may be proven valid if it needs to be enforced, it is better to have a written agreement. Some laws require written contracts, such as certain real estate transactions. Additionally, there are laws that require contracts for agreements that may last longer than one year.
A sample list of business agreements you should get in writing:
Any real estate sales contract
Commercial and residential lease agreements
Sales agreements for large purchases
Service agreements for costly jobs
Partnerships or joint venture agreements
Loan and investment contracts
Agreements that last longer than a year
Agreements that last past the death of one of parties