Reservations Necessary In May?

The major issue will be festivals. For example, Seville's "April Fair" is actually in May this year, May 4-11. Getting a last-minute hotel in Seville over that time would be difficult if not impossible. Other cities will also have their spring festivals on different days in May. This is something to look into - when might you want to be in different places and when do they have a festival.

Otherwise, just the normal advice about having internet access and booking online when you know where you want to be.

It could cost more or you won't get your preferred choice, that is all.

Generally speaking, you should be fine. As already noted, get reservations for the popular places once you know for certain where you’ll be and when. Most of the time when we visit Spain, we have a rental car and have no idea where we’ll be more than a day or so in advance. Once we know, we’ll make a reservation. We’ve had no problem finding a place to stay although we tend to visit smaller and less touristy places.

If you are traveling by train, you may want to save big time by buying tickets in advance, which locks you into an itinerary. If you want to see the Alhambra, you have to book tickets well in advance before they sell out. That again locks you into at least part of your itinerary.

I'm unclear as to why you are concerned about the weather in May.

I have always done very well reserving ahead with charming hotels and nice apartments; I have always done badly on the road with some laughably awful places that cost far too much, so I always recommend booking ahead. If you don't do that, at least have a list of places at possible stops, so you can call ahead the day before and book or even once on the road. We did that last year in France and were able to line up our lodging as we went and decided how far we wanted to get that particular day -- we just did it for some of the longer stretches where we had looser plans. Once years ago, we went through our entire list of places and they were all booked or closed for the season and we thought we were going to have to sleep in our car -- till we finally stumbled on a place kitted out like a prison cell (concrete poured platform with a foam topper for a bed). We were grateful to have anything and their restaurant was surprisingly good.

Thank you all for your helpful input! We will have a car and will only be "winging it" in Costa Brava and the hill towns of Andalusia. I'll make reservations ahead for any locations where it looks like reservations are getting tight. We usually travel in a shoulder season which I hope is to our advantage. Traveled totally free of reservations on two different trips to Croatia but wasn't sure we could pull that off in Spain. I'll give it a try and let y'all know how it goes. Thanks again!

My experience is that hotels in Ronda fill up in February and March. If you're planning to stay there, at least have a list of hotels in advance. I stayed in Ronda twice for several nights with a car and explored the pueblos blancos in different directions each day - and allowed time for all the lovely and quirky sights in Rondo too.