Tracking airfare can turn a sane person into a conspiracy theorist. Are the airlines tracking my computer? Am I being punished for booking on a Wednesday instead of a Tuesday?

The truth is airfare prices are dynamic. Pricing algorithms are influenced by long list of factors — the time you’re flying, how many seats have been sold, the location demand, etc.

You could spend an endless amount of time stalking prices, hitting refresh to watch airfare ebb and flow until you get a deal. Or you can set up price alerts to do that for you.

Price alerts use algorithms to track specific flights, routes and dates based on your preferences, then notify you as the price changes. You can make the search very specific (a round-trip flight on Delta from Los Angeles to Miami) or broader if your plans are flexible (a one-way trip to Chicago from the D.C. area).

When Carrie Bradley, editor of the family travel blog Flying With A Baby, has enough lead time before a trip, she sets up price alerts and plays around with different search configurations. To come up with more options, she said she tinkers with the number of stops, fare class, and flying into and out of different cities (“open jaw” flights).

The result? Hopefully a cheaper ticket than she would find on her own. To start tracking, here are four of our favorite sites and apps.

Google Flights is an expert favorite. It’s well known and easy to use, and it helps travelers by filtering searches, booking flights, showing flight carbon emissions and, most importantly, tracking cheap flights.

Gunnar Olson, flight deal analyst and travel reporter at Thrifty Traveler, said that of all the airfare trackers, Google reigns supreme with its calendar search feature and ability to tell if you’re getting a good deal on a given fare. The only downside? It doesn’t catch all airlines, with major omissions including Southwest.

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To start, head to Google Flights, then choose your route and additional details, such as the number of stops, your cabin class and how many tickets you need. Then turn on “track prices” to receive email updates when prices change for your dates. If your travel dates are flexible, you can choose “any dates” for more options.

“Google will also alert you if shifting your travel by one day can save you money, too,” Olson said. “Flexibility is key if you’re looking to save while traveling, and Google Flights automates that flexibility.”

Skyscanner is Bradley’s first stop for ticket shopping. To set up a price alert, search for your route and specific dates then click “Get Price Alerts,” which should appear next to the results. Once you enter your email address, you will get an email from Skyscanner to confirm your alert.

If you sign up for multiple alerts, Skyscanner will bundle them into one email so you’re not overwhelmed. For searches for flights taking off in less than 100 days, Skyscanner will send you a daily email. For departures taking off in more than 100 days, you will get an email once a week as prices tend to be more stable further out.

The mobile app Hopper can monitor airfare, as well as hotel and rental car prices, and tell users whether they should wait or buy based on its analysis of flight prices and historical data.

Once you have downloaded the app, search for your flight, then select “Watch This Trip” to get notified when prices change. If you think you see a good deal but aren’t totally sure you want to book yet, you can hold the fare with a temporary price freeze. Expect to pay a fee ranging from $5 to $40 based on how long you would like to hold the fare (you have up to a week).

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The online travel agency Kayak is a platform to search for flights, rental cars, hotels, train tickets, vacation packages and activities (such as bus tours and museum visits).

To create a Kayak Price Alert for a search, start by searching for the flight you need. Once your results appear, you will see the option to track prices along with Kayak’s advice on whether to buy now or hold off based on their projections. You can also set up a flexible alert by selecting “Flexible Dates” and choosing from options like “Anytime,” “Upcoming Weekends” or a particular month.

When your price tracker is set, you will get notified by email (or push notification if you have the Kayak app) when there are noteworthy price changes.

While less specific than price trackers, subscriptions to cheap flight alerts can help you find deals. You can sign up for newsletters from sites such as Scott’s Cheap Flights, Thrifty Traveler, Airfarespot and Airfarewatchdog to get deals based on your home airport or region. You may end up with a deal in your inbox that fits your travel needs.

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