In most places, it's required by law to have a prescription to dispense contact lenses. Prescriptions are typically valid for a year, so you may want to keep an eye on the calendar and reorder your new contacts right before yours expires. Some retailers have an online tool that will check your vision, send your results to an optometrist, and update your prescription - often at no charge! If you don't have a complicated prescription and don't think your vision has changed much since your last in-person exam, this could be a convenient option.