If you eat a variety of fruits, vegetables and whole grains along with moderate amounts of a variety of low-fat dairy and protein foods and the right quantity of calories, you don't need to use supplements.  Most Americans do not eat all the foods their bodies need every day, so a multi-vitamin and mineral might be a good idea.  Special supplements are also recommended for people who are pregnant or have nutritional disorders.

Additional Resources:
http://edis.ifas.ufl.edu/fy890
http://edis.ifas.ufl.edu/fy891
http://www.eatright.org/About/Content.aspx?id=8409&terms=supplements