Microsoft .NET Framework is a set of system components from Microsoft which powers the next generation of Windows applications, including Rapid-Pi. In order to run Rapid-Pi, you need to have the .NET Framework installed on your computer.

The .NET Framework is available for download free of charge from Microsoft's web site (the download button is at the top of the page).

When you install Rapid-Pi, the Setup program will check if the .NET Framework is installed. If the .NET Framework is not installed, you will be directed to Microsoft's web site to download the Framework.