Twitter posted to their blog today that they would begin testing Promoted Tweets that would show up in user timelines. To do this, they are partnering with popular third-party web app HootSuite.

During this testing period with HootSuite, we will experiment with where and when Promoted Tweets are shown in the timeline. Not all HootSuite users will see Promoted Tweets and those who do may see different Promoted Tweets in different places in their timeline. As with Promoted Tweets in search, we will display Promoted Tweets in the timeline when they are relevant. Similar to our Promoted Account recommendations, we use several signals to determine a Promoted Tweet’s relevance to a user, including the public list of whom they follow. We will expand the rollout only when we feel we're delivering a high-quality user experience.

Twitter originally started Promoted Tweets to appear in trending topics by certain companies and organizations willing to pay for a specific tweet to be seen. They then followed up with the addition of fully Promoted Accounts. Now once the testing phase of Promoted Tweets in user timelines is complete, it will gradually begin to appear on the Twitter website and in other apps.