Hi, we are traveling for 5 months, from Amsterdam around Europe including cruise to Istanbul (turkey) and Spain but will spend significant time in Italy. We have an iPhone 6s. What's the best prepaid data sim to use and where would I get it from in Amsterdam (preferably as early as possible after arriving). We probably need around 4/5 gb. Thanks in advance.

You will be charged roaming charges, unless you purchase a SIM in each country.
best as in what way?
http://prepaid-data-sim-card.wikia.com/wiki/Netherlands
You will need the ability to top up on line.

T-Mobile!

Fortunately the EU has put a cap on roaming charges, and will abolish them altogether in 2017: https://ec.europa.eu/digital-agenda/en/roaming-tariffs

All the countries you have tagged are in the EU, so you can buy a SIM in the first country and use it in another until your data runs out. Then you can top it up online or buy a new SIM in the country you are currently in. BTW, if your cruise includes the Greek Islands, the EU rules apply there too, and any time you are near an island you should get a 3G signal.

If you are traveling soon, the data roaming is still high until April 2016, then reduced, but still has roaming charge until 2017. Don't count on universal ability to top up online. Sometimes they require something only a resident would have as a required credential to be able to top up online.

I presume you are coming from a country not restricted by the Schengen 90 day rule.

Don't use an EU phone for data in Turkey!It isn't EU and data costs will be horrendous.
You will need to have a card that you can top up no matter here you are in Europe, something which isn't always possible.
Roberts answer of T-Mobile only applies if you are in the US. T-mobile in Europe is not as generous.

The main problem with most Dutch providers is that they only accept a Dutch system of payment online or vouchers for call and data packages.
Lebara don't have 4G automatically but they do accept credit card payments online.

As greg says I assume you know about Schengen rules.

As stated above, EU roaming rules don't apply in Turkey or any other non-EU country. When cruising around islands near the Turkish mainland, make sure that the signal your phone is receiving is from Greece (Vodafone, Cosmote, or Wind), and not from a Turkish provider. That would apply anytime you are near an international border.

You will need to do your math and decide whether to stick with one or two SIMs or change in each country. If you need to stay in contact with people back home, your contact number will change every time you get a new SIM.

It is easy to find free wifi hotspots just about everywhere you go in Europe, so you may not need as much data for your iPhone as you think. Ironically the most expensive hotels are usually the ones with high wifi charges, while budget hotels often provide it for free. If you don't already have the Skype app on your iPhone, download so you can make free video calls.

Some countries, such as Italy, already have some plans with affordable data roaming.

If you're first going to Amsterdam, I would try to find something available in Amsterdam that will work for most of your trip. I don't know anything about Dutch plans, but maybe someone else can help you with that. I looked at Lebara, but it seems that their data bundles can be used only in the Netherlands.

However, if your trip will be after April 30th, 2016, the EU will already have placed a significant limit on data roaming charges, so we should see a lot of new plans popping up. After that day, a provider can only charge the domestic price for data plus a maximum of €0.05 per mb additional. (That would be €50 for a gb, but it's a lot better than the maximum charge now, which is €200 per gigabyte.)

https://ec.europa.eu/digital-agenda/en/roaming-tariffs

If you'll be spending a good deal of time in Italy, you might want to get an Italian SIM card for that part of the trip. They do have plans for use outside of Italy that give you a reasonable amount of data. I use TIM in Viaggio Pass when I travel. They don't have a page in English, but you could run this page through translate.google.com :

https://www.tim.it/offerte/mobile/es...m-viaggio-pass

This plan has a price <b>much</> lower than the current EU maximum.

As far as I know, all Italian providers require that you have an Italian credit card to top up online. Some accept PayPal, but only if you registered with the Italian PayPal, which again requires an Italian credit card. You can make an estimate of how much you'd spend outside of Italy and top up before you leave the country, because you can activate and terminate options online, or sending an SMS to the provider.

Sorry I messed up my formatting, didn't mean to shout.

How about Spain & Portugal specifically? Arriving in Valencia, spending 10 nights in Spain, then 5 nights in Portugal.
Recommended SIM card? Need different card for Portugal? Best places to purchase one?

Thanks everyone. Great advice. We're coming from Australia and traveling in late June 2016. That you all

Dave, I don't know anything specific about Spain or Portugal, but I think it's unlikely that you need to get a second SIM card for Portugal. Will you be a heavy user of data services?

When you buy the SIM card, tell them that you'll be using it also in Portugal, because they might have options or packages that make roaming cheaper.

bvlenci, thanks. I went to ribeirasacra's link for /Spain. Vodafone has a 15 Eur deal that looks like what I need. We'll check them out when we get to Valencia and see if Portugal can be dovetailed to it.

Okay, so you will be travelling after the EU roaming cap, which makes it less advantageous to change SIMs every time you move from one country to another.

Frankly, in recent years I have seldom even bothered buying data SIMs, because it is so easy in Europe to find free wifi hotspots in hotels and cafes, sometimes even on the street. The only time recently that I've bought a data SIM was in Cape Town, SA, which has good 4G coverage, but not so good broadband where I stayed.

If you stay in Europe just for a while, prepaid service is the best solution.As long as your mobile is GSM-compatible, it shall be fine.

You can try LeFrenchMobile, they have an offer to call, text and data for all EU countries. No need to activate and really easy to top up, customer service is nice too.