Great question. It depends. Here is a simple answer:

First Class Medical:

Required for:

1. Airline Transport Pilots (ATPs) acting as the pilot in command. “PIC” in this case means manipulating the controls. It doesn’t mean acting as the Captain.
2. When operating as a SIC in a Part 121 operation that requires 3 or more crew members
3. When serving as a crew member in a Part 121 operation at age 60 or above.

If you are trying to get hired by a regional airline, you had better get one of these before you apply for the job. They will want to see a copy of it during the interview process. Make sure you don’t have some underlying medical condition preventing you from getting a first class medical before you apply to the airlines.

Some Part 135 operators will require a 1st class medical. It depends on the operator. If you are trying to get a job with a Part 135 operators, the application will say what medical level you need.

Note: The military doesn’t recognize first class medical since those are created by the FAA.  The military uses its own doctors and has strict medical guidelines. It is much harder to qualify medically for the military than the FAA. You don’t need a 1st class medical to apply for military flight training either. They don’t care.

Expiration: 

Under 40: 12 calendar months

Over 40: 6 calendar months

If you are under 40, and it expires it automatically reverts to a third class medical. If you are over 40 it will revert to a second class medical then after 12 calendar months it will revert to a third class medical.

Second Class Medical:

Required for:

1. Commercial Pilots
2. As an ATP, but acting only as a SIC of a Part 121 operation. (This is an unusual case, most, if not all Part 121 operators require Class 1 Medicals)

This doesn’t mean if you have a commercial pilot’s license you need a second class medical. It means if you are exercising your privileges as a commercial pilot you have to have a 2nd class medical.

In other words: are you getting paid to fly passengers or cargo around? Yes? Then you need to get a 2nd class medical.

Do you have a commercial certificate, but you don’t intend to get paid, you just want to fly around like a private pilot? Then you don’t need a 2nd class, you only need a 3rd class. Make sense?

Expiration:

12 calendar months regardless of age. It will revert to a 3rd class medical after 12 months, and then depending on your age it will expire after 60 or 24 calendar months from the issue date.

Third Class Medical:

Required for:

1. Private pilots
2. Recreational pilots
3. Student pilots
4. Flight instructors acting as a PIC (ie actually manipulating the flight controls)
5. Flight instructor acting as a required crew member (I don’t know who decides whether you are required or not, maybe the Part 141 school?)
6. When taking a practical test for your private, rereactional or flight instructor certificate.
7. When acting as an Examiner for a practical test.

Expiration:

Under 40: 60 calendar months

Over 40: 24 calendar months

Note: AOPA is aggressively seeking Third Class Medical Reform. Check out this article for the latest: Third Class Medical Reform.

Pilots who do NOT need a medical certificate:

I am not going to go into the whole laundry list of people who don’t need medical certificates.

Instead click this link to read a complete list:  14 CFR 61.23

Reference and additional resource:

The official regulation governing medical certifications is: 14 CFR 61.23

Want to dig deeper on this topic? Go here: Flightphysical.com.  The website isn’t very well laid out, but if you scroll to the bottom of the link I provided you will see additional pages.

Are you a sport pilot? You have different requirements I won’t cover in this article. Instead you can check out the 14 CFR 61.303.