Twitter will soon be peeking in on your smartphone. Literally.

The social network says it will start collecting data on which apps its users have downloaded onto their phones. The update is opt-out, meaning Twitter will start collecting this information from users automatically unless they explicitly tell it otherwise.

Twitter says the reason for the update is simple: It’s trying to learn more about its user base so it can make more money selling ads.

The company’s business model is dependent on targeted advertising, and while Twitter can already determine some things about its subscribers based on what they tweet and whom they follow, the information is still limited, especially compared with Facebook’s extensive set of data.

The move will also help Twitter improve the content recommendations it offers. In the past few months, Twitter started showing tweets to users from people they didn’t follow, using an algorithm to identify tweets its new users may like. Twitter also recommends people for users to follow based on who else is in a person’s network. Knowing which apps its users have gives Twitter another signal.

It could also help Twitter’s push to sign up new users. Earlier this month, the company talked about an “instant timeline” product that could help new users automatically follow relevant people. The plan is to keep new subscribers from facing a blank timeline right after they sign up. Again, knowing which apps people use could help Twitter populate that timeline.

Both Apple’s iOS and Google’s Android operating systems already allow third parties to gather this type of data. On iOS, for example, developers can ping a user’s device at any time and recall a list of apps that are currently running on their smartphone. If they ping the device often enough, the developer could piece together a user’s entire app library. A third party can also selectively seek out specific apps users may have on their smartphone — for example, does this user have Spotify?

On Android, developers can recall a list of all apps on the phone so long as they disclose their ability to do so within the app’s terms of service, according to a Google spokesperson.

That does not mean, however, that developers can gather data from within those apps, such as how often a person uses them, or what information a person has shared. Developers can only collect the app name. Twitter, for example, may know a user has Spotify, but it can’t know which songs the person is listening to.

Wednesday’s update will undoubtedly rub some Twitter users the wrong way, but Twitter isn’t the only company using this strategy.

Facebook, for instance, also collects some information about which apps users have on their phones, but only for apps that use the company’s software developer kit, according to a Facebook spokesperson. For example, if Candy Crush uses Facebook’s SDK, Facebook could ping people’s phones to determine who has already downloaded the game, helping Candy Crush better target its advertising.

Users can opt out of Twitter’s new data collection at any time within their phone settings, and the company will publish an in-app notification pointing users to the new data policy before any new app data is gathered. If a user does choose to opt out, any data previously collected from that user’s phone will be deleted from Twitter’s servers, according to people familiar with the update.

The app update will go out to iOS users today, and will surface for Android users in the next week or so. You can find more detailed information on how it works and how people can opt out here.