Cheapest flights.

Search Google flights
https://www.google.com/flights/

Beware there are a lot of other databases out there that will show a low price but the flight will involve a lot of changes and long or odd travel times For example leave at 6am arrrive at 11 pm something ridiculous

You get what you pay for.
But Norwegian air and Wow out of Iceland are hot low budget European airlines.
You just have to live on the east coast though.

I use to work for Virgin Atlantic, Tuesdays or Wednesdays were the lightest loads cheapest flights

Correction on Norwegian: Norwegian flies from Oakland, Los Angeles and Las Vegas in the west. They have pretty good fares with occasional outstanding sales. They are flying the 787 from Oakland.

Norwegian Air Shuttle also flies out of Boston, a little airport north of NYC, JFK, Ft. Lauderdale and Orlando. If you're fortunate to live close to those cities, LAX, Las Vegas and Oakland, you're in luck.

In my opinion, you should probably avoid Frankfurt as your airport destination - it seems to me that this is the most expensive place in all of Europe to fly into and out of. Like the posters above, I have been checking out Norwegian and also SAS. I see some crazy cheap fares from Oakland to Arlanda, Sweden (for example in April) for LESS THAN $500 round trip! (Not that I'm planning a trip, but I do like to dream...)

Last July I spent around $2,700 on United from SFO to Frankfurt (upgraded to Premium economy) but - sheesh - that's just too expensive. Not going to fly in July / August again if I can help it....

@ Shelley...Unerhört! .I flew SFO to Frankfurt mid-May, returning in June, FRA to SFO, the cost was $1500 plus but in Economy (i Know the 11 hr sardine can).

Contrary to an opinion above, I've found for us that Frankfurt (airport code FRA) is often the cheapest arrival airport in Europe. One must go through many and seemingly illogical permutations to get the best airfares, being mindful also of nuisance and time considerations.

We have been using Aer Lingus, mostly because they just started a direct flight from our home airport (Hartford - Bradley) to Dublin. We booked round trip flights to Dublin in May 2017 for $506. Last year we went on the RS Sicily tour, and used Aer Lingus to fly to Rome via Dublin. We stayed in Rome for a couple of days before taking an Alitalia flight to Palermo. At the end of the tour, we flew on Alitalia from Catania to Rome, stayed overnight, and then flew home. Using Aer Lingus saved so much money compared to other airlines that it paid for the extra days in Rome. Aer Lingus allows you to go through US customs and immigration in Dublin on the way home.

Norwegian just started some flights from Providence, Stewart (Newburgh, NY) and Hartford to various destinations in Ireland and Scotland.

Probably the widest range of airlines in and from North America is covered by Itasoftware, a Google company serving airlines and not selling retail tickets. It shows many flights and prices, information to be used with searches on other sites. www.matrix.itasoftware.com
Many factors are in play -- non-stop flights versus connections, amount of competition on the specific routes, world events such as terrorism, fuel prices and fluctuations in currency exchange rates, surcharges at individual airports. Demand often varies according to season, and airline planning for the capacity needed on each route may hit the target or not. The summer of 2016 saw expanded capacity from eastern North America, especially Canadian carriers, which led to empty seats and some discounting. So far this year the prices being charged for spring and summer appear to have risen back to the level of 2015 if not higher. There will always been exceptions on specific routes and for particular times. The massive computers at the heart of air travel shift the situation as often as the ocean tides. Good luck to you in gaming the system.

Fly off season. (It would help if joeyzedeck gave details on what he's looking for but...) Be flexible on dates and even airports.

I got RT Atlanta to Madrid leaving Wednesday for $466 during a sale last fall. I have RT in Premium Economy from Orlando to London in October for $800 (This one was higher due to some date restrictions. I could have gone cheaper if I hadn't had to fly home on a Monday, but real life gets in the way sometimes!)

Watch for sales. Here, TripAdvisor, Fare deals on Flyertalk, etc... and be ready to move! I posted one on Business fares this week that was gone in HOURS!

Okay,a few here helped a lot. I should have Been more specific. I want to fly out of either Chicago or Detroit (given I live in Michigan) to Madrid which is where I'm staring my trip. A lot of things can change because this is in June 2018 but am I better of buying really early or trying to find a deal later on. Thanks

Joey

Look at thte Matrix website suggested by Southam 3 posts above. You can only input dates less than one year ahead, so it is too early for June, 2018.

However, you can input for June, 2017 to get a good idea what to expect what a year from now. When i did a quick check for a 2 week trip any time in June, 2017, flying round trip out of Chicago to Madrid seems to be cheaper than out of Detroit. Best airline seems to be Iberia at about $1042 pp for an 8-9 hour non-stop flight each way. You can get as cheap as $842 pp but then you have multiple stopovers and will take you 20 to 73 hours each way.

I don't know about you but i would spend the extra $200 and take the direct flight.

So right now we are about 2.5 months away and there are still flights available. You can follow the prices over the next few moths to see how they change as you get closer to June 2017. Who knows for sure what it will be like one year from now. And i have no idea if the price for June 2017 was cheaper or more expensive 2 or 3 months ago. But as the date gets closer, the fares will probably become more expensive and you risk seats being sold out. (We booked flights to Big Island Hawaii last month for mid April. Same flight today is almost CAD$200 more today). I advise not to gamble on airfare. You can always eat cheap or sleep on a park bench to save money on your vacation. But no flight availability means no vacation.

Sky Scanner is another tool for airfare searches.

You can load multiple departure cities along with multiple arrival cities in one selection and get alerts from each departure to each arrival. The alerts tell if the price is going up or going down.

This tool save me more than $400 on my flight to/from Rome later this summer.

https://www.skyscanner.com/

It's way to early to book flights for June 2018. I would start tracking prices maybe 9 months out, and do this for a few months. You're not going to get a deal any earlier because airlines algorithms are constantly predicting future demand to set pricing, and most people don't have their preferences set or bookings completed this far in advance (so there's no data to set solid prices yet). Also, keep in mind that the earlier you book, the more likelihood that you'll have to deal with multiple schedule changes as airlines also tend to make small (or sometimes substantial) schedule shifts, again aligned with predicted demand.

Fares are extremely specific to origin, destination and dates, so the only thing that matters is Chicago-Spain and Detroit-Spain during your timeframe. I don't think you'd save a significant amount of money by flying into a city other than Madrid, but I suppose it wouldn't hurt to keep an eye on Barcelona and Malaga, just in case something wild happens.

Last year was pretty strange for fares on many US-Europe routings. From September through November 2016 there were frequent spottings of incredibly good fares for spring/summer 2017, seemingly driven by low demand (perhaps terrorism-related??). There's no way of knowing what will happen this year for your itinerary, so I recommend tracking fares on the best 2 or 3 flights (time-wise/location-wise) as soon as your dates become available. Check as frequently as can (daily is not too often). That way, if there's a sudden, substantial drop, you'll know it's an exceptional deal and can act right away. The low prices sometimes come and go within 72 (or even 24) hours.

Some airfare sites (including Google Flights, which I use) allow you to monitor fares on a specific flight. You'll get emails periodically when the fare changes. To me it's not a substitute for doing frequent manual checks, but it is a backstop/reminder in case you become busy and get out of the habit of checking yourself. It's easy to let life get in the way when your vacation is still 6 or 8 or 11 months in the future. It was posts on this board that tipped me off to what was going on last fall, and that allowed me to snag flights at about half the usual fare.

Thank you guys. This was very helpful

WOW also flies out of San Francisco. We just flew it r/t Paris. It was very good. Wonderful flight attendants. Very low airfare. Customer service not good though.
Not useful info for you Joey, but might be for someone else reading this.

Madrid's a good choice. I have seen several good deals there. I think it's not as on the "radar" for US tourists as the biggies (London, Paris, Rome for example)

The fare I have to Madrid next week is on Delta so there probably were some deals out of DTW. But.. June's tougher!

Good Luck!

And don't forget that Norwegian Air Shuttle just doesn't fly to London, Oslo and Copenhagen. Get to their gateway cities and they have a large network of connecting flights all over Europe.
We're flying one way from Budapest on 4/26/2017 for $238 each into Boston.

And everything about the "new" website just posted screams "run". Go over to the Air Travel forum on Trip Advisor. At least once a day there's an entire thread about someone who went to one of these great saving websites and got completely ripped off. Be careful!