VST/ AU plug-in doesn't appear in Live

Live Versions: All
Operating System: All

In case a plug-in is not showing up in Live's plug-in folder, please take the following steps:

Ensure you have the correct versions of the plug-ins installed

32-bit plug-ins will not show up in the 64-bit version of Live, and vice versa. Ensure that you install the correct version of the plug-in depending on which version of Live you are using.
Some modern plug-ins are only available in 64-bit (such as Kontakt as of version 5.1). If you are using the 32-bit version of Live, these will not show up.
Some older plug-ins are only available in 32-bit and will not show up in the 64-bit version of Live.
It's not recommended to store the 32-bit and 64-bit versions of plug-ins in the same folder.

Update the plug-in

Check the manufacturer's website and make sure you have installed the latest version of the plug-in.

Ensure you have installed the plug-in in the correct location

Check the default installation locations here:

Perform a deep rescan of the plug-ins folder

Hold ALT/Option and press "Rescan plug-ins" in Live's preferences in the File/Folder section. This will perform a deep rescan of all installed plug-ins.

Make sure you are installing a VST 2 plug-in - VST 3 is not supported

VST 3 is not supported on Live. If you are trying to install a VST3 plug-in, check to see if there is a VST 2 version instead. If on Mac, there may be an AU version available.

Don't move the standalone or VST versions after installing (Win only)

VST plug-ins might stop working when the stand-alone application of the plug-in gets moved, renamed or uninstalled.
It's recommended to always install VSTs directly to the correct destination folder, rather than manually moving the DLL file afterwards.

Use Admin rights (Win only)

Certain plug-ins require Live to run with admin rights. For example plug-ins which offer the possibility to drag MIDI clips from the plug-in into Live, like Toontrack's EZDrummer.