Visitors from the USA or Australasia usually have to fly via Asian or other Middle Eastern hubs. Most flights land at Tehran's Mehrabad airport. However, the situation with the Iran-Armenian border is not clear, the border with Afghanistan is currently closed and independent travellers are not allowed to cross into Iraq from Iran. If you're bringing in your own car, be prepared for major delays trying to cross the border - plan ahead and bring loads of patience. Although Iran's transport system isn't as developed as those in western countries, it's considerably better than most other countries in the region.
All public transport is frequent, reliable, relatively safe and very cheap.
There are lots of different bus companies offering competitive and mostly comfortable services. Transport can be a problem for about 10 days before and after No Ruz, the chaotic Iranian New Year which starts on or about 21 March. Road travel can be interrupted by roadblocks at any time of year, most frequently on either side of a main city, but occasionally dotted through remote areas for no particular reason. Foreigners are rarely hassled at roadblocks - the worst that will happen is that you'll have to show your passport and endure a delay. Driving your own vehicle is a gutsy call.