What is controlled airspace?  After all, a lot of “controlled” airspace is actually not under direct control at all, right?
Well let’s start with the definition right out of the Aeronautical Information Manual (AIM):

3−2−1. General

a. Controlled Airspace. A generic term that covers the different classification of airspace (Class A, Class B, Class C, Class D, and Class E airspace) and defined dimensions within which air traffic control service is provided to IFR flights and to VFR flights in accordance with the airspace classification.

Ok, so now we know the basics of what qualifies as controlled airspace.  It has to be Class A, B, C, D, or E and air traffic control has to be able to provide services.
So what services do you get?

c. IFR Separation. Standard IFR separation is provided to all aircraft operating under IFR in controlled airspace.
d. VFR Requirements. It is the responsibility of the pilot to ensure that ATC clearance or radio communication requirements are met prior to entry into Class B, Class C, or Class D airspace. The pilot retains this responsibility when receiving ATC radar advisories. (See 14 CFR Part 91.)
e. Traffic Advisories. Traffic advisories will be provided to all aircraft as the controller’s work situation permits.
f. Safety Alerts. Safety Alerts are mandatory services and are provided to ALL aircraft. There are two types of Safety Alerts:

1. Terrain/Obstruction Alert. A Terrain/Obstruction Alert is issued when, in the controller’s judgment, an aircraft’s altitude places it in unsafe proximity to terrain and/or obstructions; and

2. Aircraft Conflict/Mode C Intruder Alert. An Aircraft Conflict/Mode C Intruder Alert is issued if the controller observes another aircraft which places it in an unsafe proximity. When feasible, the controller will offer the pilot an alternative course of action.

Now we can see that controlled airspace actually gives us a lot of services if we want to use them.  While most of these services are dependent on your altitude and the availability of radar in your area, they are extremely valuable in high-density airspace and areas where you may not be familiar with the air traffic.
Air Traffic Control has to give us traffic separation services to ensure there are no conflicts.  ATC must also provide low altitude alerts.
The price for all of this is small.  If you are IFR you have to file and activate an IFR flight plan.  If you are VFR, you just need to call the appropriate ATC facility and request advisories.
There are a few more limitations on what you can do in controlled airspace.  Most of these are pretty specific types of operation, but good to know!

3−2−2 Controlled Airspace
g. Ultralight Vehicles. No person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from the ATC facility having jurisdiction over that airspace. (See 14 CFR Part 103.)
h. Unmanned Free Balloons. Unless otherwise authorized by ATC, no person may operate an unmanned free balloon below 2,000 feet above the surface within the lateral boundaries of Class B, Class C, Class D, or Class E airspace designated for an airport. (See 14 CFR Part 101.)
i. Parachute Jumps. No person may make a parachute jump, and no pilot−in−command may allow a parachute jump to be made from that aircraft, in or into Class A, Class B, Class C, or Class D airspace without, or in violation of, the terms of an ATC authorization issued by the ATC facility having jurisdiction over the airspace. (See 14 CFR Part 105.)

Those are the basics of controlled airspace.  Of course there’s a lot more we’ll be covering in upcoming sections!
Please post your questions and comments below!