Minor dents are often just filled, bad ones require door replacement if available. If they're alum. re-skinning/rebuilding an existing door may also be an option. A door frame that's been wiped out, or a door that's all fiberglass, can be a real challenge if no longer available - especially those radiused to match body lines.
By far though, getting the paint to match after the repair is the most difficult part of the job. Amateur "close" matches can be unsightly.... Often, just leaving the dent would have looked better.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake