An advance directive allows a person to document their wishes for medical care and appoint a healthcare agent if they become unable to make decisions. They can take the form of a living will to specify treatment preferences or a medical power of attorney to appoint a surrogate. Anyone over 18 can create an advance directive, which provides clarity for loved ones and ensures one's preferences are followed even in an emergency. States have different requirements, so it's important to understand applicable laws. Advance directives can be changed at any time if capable, and reviewing them periodically is recommended.