Optimizely Content Management System (CMS 11) is built on ASP.NET 4. It is an "everything-in-a-box" type of product, and it is simple, easy to use, and familiar to users. CMS 12 is built on ASP.NET Core, which has a lot of differences from ASP.NET 4, but fundamentally it is still the same product - everything-in-a-box, simple, easy to use, and familiar.
Apart from the underlying framework where the APIs were migrated from ASP .NET 4 to ASP.NET Core, CMS 12 and CMS 11 have a one-to-one behavior and syntax by using the same architectural patterns. You can still build everything in one application as a developer. Nothing changes in how your website looks, how it is structured, or how the editors work.
### Why upgrade from ASP .NET to ASP.NET Core?
Microsoft is no longer investing in the ASP.NET 4 stack.
[**.NET 5 performance** is impressively fast](🔗) compared to ASP.NET 4.
**Ecosystem** - ASP.NET is a popular framework for server-side programming, APIs, and integrations. Partners want this and with CMS 12 and Commerce 14, they are getting it.
CMS 12 supports more **headless-style deployments**.
## Versions of CMS 12
CMS 12 comes in the following versions:
**CMS 12**. The core platform built on ASP.NET Core.
**ASP.NET SDK** for headless implementations.
**React SDK** for implementations using React.
**Vue.js SDK** for implementations using Vue.js.