Third-party (AKA cross-site) cookies are harmful to the web, and must be removed from the web platform. This finding explains why they must be removed, and examines the challenges in removing them. We highlight some use cases that depend on third-party cookies and offer some examples of designed-for-purpose technologies that can replace them. Specification authors are expected to ensure they do not undermine the benefits of removing third-party cookies when proposing new web platform technologies.