PayPal helped invent online checkout. Nearly three decades later, it's struggling to defend its turf.

The iconic online payments company is facing its biggest challenge in nearly three decades of existence. Its core business of customers using the app to check out when shopping online is barely growing and new management has bluntly warned investors that "significant changes" will be needed to fix the company's problems.

One of the biggest success stories of the original dot-com era, PayPal has seen its territory steadily conquered by new and existing competitors, particularly Apple, Shopify, the buy now, pay later companies like Affirm and Klarna, and peer-to-peer money transfer services like Cash App and Zelle, particularly in the past five years.