Shortly after the launch of Apple Pay, a number of retailers, including Walmart, Rite Aid, and CVS, blocked Apple’s tap-to-pay technology from their stores. Instead, these retailers banded together to place bets on an in-house mobile payments system called CurrentC. That network was developed in partnership with Boston-based mobile wallet technology startup Paydiant.

With news today from Mobile World Congress that PayPal is acquiring Paydiant, PayPal has officially become a thorn in Apple’s side. One of the reasons CurrentC has been so appealing to retailers is Paydiant’s technology can track merchants’ rewards programs and offers, something not yet available on Apple Pay.

So far, payment systems have been a major theme at the tech show, which is taking place the week before Apple reveals more details about its upcoming smartwatch—an event where payments will undoubtedly play a prominent role. On Sunday, Samsung debuted its new flagship phones, the Galaxy S6 and S6 Edge, at the show, along with a new payments platform called Samsung Pay. Google also revealed a new developer tool called Android Pay.

Though PayPal is a giant in payments processing, it has relied heavily on strategic acquisitions, such as Braintree and Paydiant, to stay relevant in mobile. The company has had limited success with moving into physical stores, but it’s hoping to change that with Paydiant as well as a new credit card terminal equipped with Near Field Communication that will roll out in the UK and Australia this summer, and in the US later this year.