US, British & Canadian citizens
Contrary to popular misconception, US & British citizens are welcome, but need to pre-arrange a tour or private guide, or be sponsored by a friend or relative in Iran, who will take legal responsibility for them. Rules that applied earlier only to Americans have been changed since February 2014 and extended to include British and Canadian passport holders as well.
Canadians, British and Americans need to be accompanied be a guide while they are in Iran. Contact a tour operator to discuss an itinerary. In practice, nobody checks if the tourists are accompanied by the guide once inside Iran, this happens only at the border.
US citizens can apply for a visa at the Iranian Interest Section of the Pakistani Embassy in Washington. However, US citizens must have an MFA-approved guide to accompany them for the entire trip and must have an exact itinerary. Only when the itinerary is set, the travel agency may apply for a visa authorisation number from the Iranian Ministry of Foreign Affairs. Once approved, the authorisation number is transmitted to the interest section. At that point, the applicant can then apply for the visa.This generally precludes crossing into Iran at any border, as your guide would have to meet you at the border. Tour guides, however, are generally friendly to Americans, understand the process, and can work with you to set up a custom itinerary for you.