Do I need a re-entry permit when on a ME Non-B visa in Thailand?

If you are on a Multi-Entry Non-B visa, you must obtain a re-entry permit each time you leave Thailand if you wish to return without invalidating your visa. Although you can leave the country every 90 days and re-enter, doing so without a re-entry permit will not 'reset' your duration of stay, and your visa could be rendered void. Therefore, it is important to secure the appropriate re-entry permit before departing.