Prague in Late May

It is always crowded. 6.2 million tourists is just something that we have to deal with. Get up early in the morning and enjoy the city centre before the hordes arrive.

The main advantage of May is that it is cooler. The hot weather tends not to arrive until July and August, although June was quite hot this year.

Wife and I were in Prague from May 31-June 3 last year. The only place the crowding was unbearable was right in front of that amazing clock in the old town square, when it was putting on its show. Other than that, the square itself was busy, but not bad; we never had any trouble finding a nice table for beer and people-watching (and the acts on the stage in the square were good to great). The Charles bridge was too busy for our taste, but then we only used it to walk across and not hang out; our experiences at the Kafka Museum, the Museum of Communism, the castle, etc., were not compromised at all by crowding.

Prague in May is in its best. Pleasant weather, parks in blossom. Looking across the river you see Hradcany Castle and left of it Petrin Hill fully in bloom with its little Eiffel Tower on the top. As Marcus said - Prague is always crowded, summer the most, winter the least. If you want to enjoy Prague without crowds do your sightseeing early morning.

We were in Prague May 14-20 this year. It was our first visit and we stayed for a week in the 7th district (i.e., well out of the tourist areas) which was easily accessible by tram and/or metro. We loved Prague and had a wonderful time! The weather was glorious for us. We got near the Charles Bridge - once - and never walked across it. We walked through the Old Town Square - once - and never returned there. We consciously chose to miss some of the "must-sees" for more obscure sights, and we had more than enough to occupy our time, mostly without having to interact with the tourist hordes. Hint: to enter the castle, go through the Royal Garden. We did this three times and never encountered a line at the security checkpoint, whereas at the main castle checkpoint the line was at least a block long. (Not to mention that the Royal Garden is beautiful.) When you're a tourist yourself, it doesn't seem right to complain about crowds of tourists (!) - but we all do it don't we?!?

May is a good time to visit and as others have said not as crowded or as hot as the main summer months, the outdoor spaces in bars and restaurants are open as are the wonderful beer gardens of the city. There is also usually a music festival on in May and there has been a big Beer festival on for the last 10 years or so.
Prague is a wonderful city and though the main tourist areas can get very crowded it is very easy to get away from the crowds and enjoy the city in other ways.