HAPI is a standard for serving time series data. It was developed with Heliophysics in mind, but has been kept as generic as possible and is likely useful in other areas of solar system exploration and astronomy. We present the basics of the standard (RESTful request and response interface with a streaming data format), and show how it has been adopted by various data centers in Heliophysics and Planetary Science, both in the US and Europe. We will also describe the various clients that are available for reading HAPI data. We aim to dialog with the astronomy community about the relevance of HAPI for serving time series data at various astronomical archives, as well as use of HAPI by astronomy software as a way to gain more uniform access to Heliophysics and other data sources.