Which is the Hot wire and which is the Neutral wire if the wires you see are a Brown a Blue and a Yellow-Green wire?

Answer for countries in Europe and other world areas running a 50 Hz supply service.
It depends on how the wires are hooked up.

In general, the accepted practice is to use the green/yellow wire for ground. The blue wire is neutral. The brown wire is hot.

But saying this does not guarantee that the previous wire monkey did it right, especially if they followed the advice left here earlier... Click on the View Discussion button below this answer to see the first answer that was given to this question! So make sure you test your wires before using them to power a new circuit.

Cheap circuit testers are available at any local hardware store for just a few dollars. Remember that a light bulb isn't a tester - it will still light up if the wires are crossed.

This could mean that your light FIXTURE is HOT - potentially killing you when you go to change it. Even switching off the lights will not remedy the situation, since you're breaking the circuit to GROUND. That means you have a floating Live wire in the fixture, "waiting" for you to touch it and complete the circuit to Ground.

So, the moral is: DO NOT rely on colour-coding. Test your wires with the right equipment.

<><><>

Which leads to the follow-up moral: Make sure you get your work inspected or have it done by a qualified and licensed electrician.

NOTE: the above "accepted color coding" where BLUE is the neutral is generally in Europe and countries other than the USA. IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.If you do this work yourself, always turn off the power
at the breaker box/fuse panel BEFORE you attempt to do any work AND
always use an electrician's test meter having metal-tipped probes
(not a simple proximity voltage indicator)
to insure the circuit is, in fact, de-energized.