Decidim (decidim.org) is a digital platform for participatory democracy, built entirely and collaboratively as free open source software. More specifically, Decidim is a web environment (a framework) produced in Ruby on Rails (a programming language) that allows anyone to create and configure a website platform to be used as a political network for democratic participation. The platform can be deployed by any organization (local/regional/national governments, associations, universities, NGOs, or cooperatives) to host large scale citizen participatory processes for strategic planning, participatory budgeting, public consultations and collaborative policy-making. The project was launched in 2017 in Barcelona and, 4 years later, there are more than 400 active instances of Decidim around the world. In this talk we will present the lessons from this outreach process advocating for a radical model of participatory democracy in contexts as different as the Helsinki City Council, the Mexico City Government, the New York City Commission on Civic Engagement, the French National Assembly, the Italian Government or the European Commission.