Do we need to buy tickets ahead of time?
When we visited the Paseo del Prado, we waited for the free admission hours. We listened to Rick Steves audio tour of the muesum and was able to see so many great pieces of art!
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You don't *have* to buy tickets in advance though there can sometimes be queues These are partially created by the security scanning, which of course is used for those with advance tickets as well.
If you do decide to buy tickets online in advance it's best to use the official site to avoid agency charges:
Note that it is possible to visit the MUSEO NACIONAL DEL PRADO without paying anything at all (I visited every evening when I was in Madrid). The Prado is free to enter from 6pm to 8pm from Monday to Saturday and from 5pm to 7pm on Sundays. The queue may initially seem long but once 6pm arrives it moves very quickly indeed. :-)
No, you do not need to. But if you do you can skip the line...
You don't "need" to, but it definitely helps. I always recommend buying tickets to any attraction in advance whenever possible. Why waste your time queuing for tickets when you can arrive with your ticket in hand and go straight to the door?