Before you receive the takeoff clearance, Tower may issue you specific departure instructions. These instructions tell you what to do immediately after takeoff: which heading to turn to, when to turn, or which SID to fly. A SID is a Standard Instrument Departure—a published procedure that gets you from the runway to a specified waypoint, usually following a specific flight path for noise abatement, obstacle clearance, or initial separation purposes.

When a SID is included in your IFR clearance (which is usual in a controlled aerodrome), the words "climb via SID" will appear in your clearance. This means:

Controllers rely on SIDs to establish initial separation and to protect you from terrain obstacles. Flying an incorrect SID can result in loss of separation from other traffic or a dangerous terrain encounter. If you are unsure of the SID, or if you cannot find it on your chart, say so before takeoff. This is far safer than departing with uncertainty.