This note is designed to be read alongside the companion policy note on electronic signatures, which explains the role of PKI in implementing cryptographically secured digital signatures within a national electronic signature framework. The present note extends this analysis through practical guidance on implementing PKI at scale, ensuring government efforts complement private sector investments. By surveying different approaches, the note showcases diverse strategies that policymakers can draw from when designing and building their own national PKI ecosystems. This analysis is illustrated through illustrative case studies drawn from countries and organizations across various regions and income levels, namely India, Brazil, the United States, South Korea, Estonia, Saudi Arabia, The Netherlands, France, the United Kingdom, and Lebanon, as well as the European Union and the International Civil Aviation Organization (ICAO).