The buzz around PASS and Serverless: easy scaling of your server side code, removing worries about provisioning the VMs your code runs on comes from helping developers move fast from code to running services. But how do you design apps that can run in those environments, and be easily ported to self-hosting when that day comes? The Twelve-Factor App methodology (https://12factor.net/), created by engineers with experience of delivering apps on Heroku, is a "recipe for success" when authoring code for these environments. In this presentation we will look at what the Twelve-Factor App methodology is, and demonstrate how to meet its requirements when creating .NET applications. We will show examples with Service Fabric and Azure Functions, and look at their evolution into self-hosted .NET Core apps, as and when you need.