This document outlines 10 lessons learned from Europeana's experience developing APIs and transitioning to being an API platform. Some key lessons include: APIs need more than just technology to succeed and require activities to promote adoption; showing concrete applications is important for gaining organizational buy-in for APIs; copyright issues can significantly hinder API adoption if not clearly addressed; and marketing and outreach are necessary for developers to learn about and use a new API. The document advocates for careful documentation of data, user experience design for developer portals, and accepting that failures will occur in the process of learning.