Where is wild camping possible in Europe?

In Finland/Sweden/Norway/Iceland you are allowed to do this by law as long as you stay away from houses and use common sense like you say. It is literally translated as 'every mans rights' (See also this question on Travel-SE for more information). In the summer these countries are perfect for this kind of holiday and I can recommend it to you very much.

In The Netherlands it is not allowed to camp in the nature (at least not during the night) as far as I know, so it is best to find a camping. I assume it is similar in Belgium and Germany but they do have more nature so I could be wrong.

France

Wild camping is allowed except in specific protected places, depicted here:

Anyway there are many local rules, so the best is to find the city hall of the village where you want to camp and ask there. An alternative to city hall is church, where the priest might be able to tell you whether the local church owns some field in the country side.

If you can read french, or don't mind asking google to translate for you, check le-camping-sauvage.fr site.

In Scotland, UK you are legally allowed to camp on privately owned fields that don't have crops. Scotland does not have a law of trespass - although camp on military land at your peril. The rest of the UK (England, Wales and Northern Ireland) do not have this freedom, you legally have to go to campsites.

In Ireland you do not have the legal right to roam (like Scotland), and legally must go to campsites.

I've investigated related topic for the bicycle trip over Italy, and I can say this:

1. There are many camping zones where you can easily stay for the night and even get a tent for rent. You can easily find such camping by internet, and choose ones matching your criteria.
2. About tolerance - remember one thing: If there any fence, rope, metal wire or something like that - it is a private territory. And you should find the owner of this zone and check, is the owner comfortable with your staying on his territory.

Wild camping is not allowed in Germany; you have to go to a campsite. From what I have heard, it's more tolerated in Eastern Europe, and no problem in Norway and Sweden.

There are many places in Spain where you can camp, on the beach for example, if you are careful about bringing drinking water. Friends have travelled across Spain without ever stopping at a formal site. The small villages in the Sierra Nevadas will mostly allow you to sleep in the vicinity, and it is possible that this is also true for the rest of Spain. This map, in Spanish, seems to provide a more formal list of pleasant areas.

For the most part, this is legal. There are certain restrictions, such as not camping near paid camp sites or in urban areas. Military areas need to be avoided as well. Lighting fires is forbidden most places for a lot of the year. Sleeping in the open air is the easiest, because you cannot just set up a tent and leave it there.

There can be problems with gangs of thieves, though I have not encountered them in the areas that I visit around the Sierra Nevadas.

In Switzerland it is officially not allowed, but if you behave normally, it is widely tolerated. Especially during summer it is not a big problem to build up your tent by a river or somewhere else. If you get caught by a police officer it really depends on the person. Most of them are very friendly and won't forbid it, but it can happen that you're forced to move on.

In the mountain regions it is different. There you are allowed to make a bivouac.

In Poland camping in the wild is tolerated, especially in the mountains. Just make sure that you camp outside the National Park (where camping is prohibited and the law is enforced), and behave well. For camp fires, choose spots that have been used before and stay away from the trees. Camping is strictly prohibited not only in National Parks, but also in every forests (and in distance 100m from trees). There are some places designed for camping and/or making fire in public forests, but they are not easy to find - here is a map.

I've been living in Poland for over 16 years and lead survival camps and do some "Solo Survival" in the many forests and mountain ranges. Technically speaking any camp fire away from designated areas is illegal but, if you're sensible and keep it modest - well, you know the rest : )

As regards wild-camping, simply follow the rules of common-sense - small, modest group; leave no mess and keep the volume low - I've yet to have a problem. In fact, a few weeks ago, with a group of 7, we camped by the edge of the forest and had a nice fire - the police visited at 2 am, saw we were causing no harm and wished us a delightful night and a delightful trip : )

In Estonia it is allowed/very much tolerated! Estonia has a very low population density in rural areas and a lot of very beautiful spots to pitch a tent for a night or a couple of days. If you're on private land, you should always ask the owner of course but Estonia also has a lot of Riigimets (state forest) where you can freely camp, pick berries and mushrooms.

In Switzerland it's not allowed but while hiking in higher altitudes it's absolutely ok. Often farmers even let you sleep in their sheds or pitch your tent on some nice even ground. When hiking even more remotely, you just tent wherever you find some even ground without worries.

Here's a Guardian article about wild camping in Europe.

There are a lot of good answers to this question. However, an answer that actually provides a complete overview is lacking. I recently wrote an article on my website where I covered all of the wild camping rules for every single European country. You can find that article here: Wild camping in Europe: Complete legal overview.

Covering every European country in a single stackexchange answer seems a little bit crazy (I needed 3500 words to cover everything). However I will provide a general summary for every region in Europe in this answer.

Northern Europe

Northern Europe is generally very wild camping friendly. There is only one country in Northern Europe where wild camping is not legal, Denmark. But even Denmark provides plenty of alternatives. Certain countries in this area of the world do not only legally allow wild camping, they even encourage it.

Central Europe

Central Europe is not as wild camping friendly as the northern countries, but most countries will at least allow a planned bivouac (one night).

Eastern Europe

In Eastern Europe, there are not really a lot of laws in place that prohibit wild camping. In these countries they generally don’t have any issue with this. Additionally, there is so much wilderness in these countries that they couldn’t possibly patrol the entire area. Note that this only applies to Eastern Europe, not necessarily to Southeastern Europe.

Southeastern Europe

Southeastern Europe is not as wild camping friendly as Eastern Europe. There are a couple of laws in place that prohibit wild camping. However, in most of these countries, wild camping is tolerated even when it’s legally not allowed. Just make sure to avoid touristic places, beaches and sometimes National Parks.

Western Europe

Wild camping in Western Europe is definitely not that easy. It’s illegal in most countries and only tolerated in a few of them. This isn’t really surprising since this part of the world is very densely populated and there isn’t that much nature left.

Southern Europe

Southern Europe is probably the least interesting place in Europe to go for wild camping. In most places it’s illegal, and more often than not, it’s also not tolerated.

Disclaimer

Note that finding legal information about every single country is not easy. Not everything in my article might be 100% correct. I might assume that wild camping is legal because I couldn't find any restrictions online + multiple people claimed that it's legal. I'm trying my best to provide a solid and correct overview, but some information might not be completely correct. If you find anything that you don't agree with, please let me know. I mean to optimize this article as good as possible over time.