I first came across the application programming interface (API) directory, ProgrammableWeb, in September 2005, shortly after it was launched by John Musser. At the time, the directory had just 54 listings. How times have changed, because nearly 16 years later ProgrammableWeb lists over 24,000 APIs. It’s an indication of how pervasive APIs have become on today’s internet. There are even API providers worth tens of billions of dollars, such as Twilio and Stripe. It’s a massive market, primarily because APIs enable developers to plug in virtually any specialist service they require for their applications. You don’t need to build your own communications service, for example — just use Twilio. Likewise, tens of thousands of applications use Stripe’s APIs to manage payments.