Google Flights provides a simple user interface to find cheap flights. It allows the user to search and filter flights by destination, dates, cost and a number of other factors.
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Pro Ability to compare price for many dates at once
You can pick the cheapest date to fly easily by comparing the different dates at once.
Pro Supports multiple departure and destination airports in a single search
Pro "Explore" feature helps you discover new (cheap) places to go
Google Flights offers a neat explore feature that allows you to search by dates, continents and interests to discover new destinations.
Pro Train connections search and alternative route suggestions (Europe)
Google Flights allows to search for train connections and suggests when travel by train is a good alternative to air travel for your destinations. You can search train connections for 56 destinations and nearly 400 routes across Germany and Northern Europe.
Pro Google integration
It's integration into Google (example: query "flights to Sydney") makes it convenient to use.
Con Not able to book directly on Google Flights
You can't make a purchase/booking directly on the site. You are instead redirected to the company offering the service and must book there.
Con Limited to flights only
Google Flights only provides information for flights, and does not include other categories that many travel search engines do (such as hotels and packages).