Additional insured endorsements are probably more important than hold harmless agreements.  These endorsements are additions to an insurance policy which have the effect of adding the name of the endorsement holder to the list of insureds under the policy.  A hold harmless agreement is a separate agreement between the parties; an additional insured endorsement is a modification of an existing contract between the insurance company, or risk pool, and the insured person or organization.  For example, the landlord of the building where you are planning the special event may require that he or she be added as an additional insured under your insurance policy.  If your special event and the landlord are then sued as a result of an accident during the special event, typically your policy will be the primary insurance covering the accident on behalf of both the special event and the landlord.  Here again, we can view this issue in reverse and see the value of requiring that the special event be named as an additional insured on the policies of vendors and cosponsors.  If the special event is sued as a result of the activities of the vendor or cosponsor, then the special event will be protected by the insurance policies issued to these other persons, businesses or organizations.