Compensation is usually due when the airline's own staff strike.
Check your flightThere is a crucial distinction here that airlines rarely explain. A strike by the airline's own employees, its pilots or its cabin crew, is generally not an extraordinary circumstance. The Court of Justice held in Kruesemann that industrial action arising from the airline's own staffing decisions is part of the normal exercise of its activity.
So if your flight was cancelled because the airline's pilots walked out, compensation is normally due, even though the airline will very likely tell you otherwise on first refusal.
A strike by people the airline does not employ is different. Air traffic controllers, airport ground handlers, security staff, and baggage handlers employed by the airport are outside the airline's control, and disruption caused by them usually does block compensation.
The test is not whether there was a strike. It is whose staff were striking. Establish that first.
Compensation is a flat amount set by distance, not by ticket price: €250 up to 1,500 km, €400 for longer flights inside the EU and for 1,500–3,500 km, and €600 beyond 3,500 km. The UK equivalents are £220, £350 and £520.
Distance is measured in a straight line from your original departure airport to your final destination, which on a connecting itinerary often lands you in a higher band than you would expect.
Duty of care is separate from compensation and does not depend on the cause. Once you are delayed two hours or more the airline owes you meals and drinks, two phone calls, and a hotel plus transfers if you are stuck overnight. This applies in bad weather, during ATC strikes, and in every other situation where no compensation is payable.
Airlines frequently provide none of this during major disruption. Buy your own, keep every receipt, and claim the cost back afterwards.
Work out what you are owed