Which CPUs does this board support?

It has an LGA1151 socket on Intel's 300-series chipset, so it takes 8th and 9th generation Intel Core processors. Older 6th and 7th gen chips will not work.

Will it fit a standard ATX case?

No. It is built for the ProDesk 400 G5 small form factor chassis, with HP's own mounting and power connectors. Plan on reusing the original HP case and power supply.

What memory does it take?

It uses DDR4 DIMMs. Match the speed and capacity to what the ProDesk 400 G5 shipped with, and check that the modules are non-ECC desktop memory.

Does it come with a CPU, cooler or Windows license?

No. The board is sold on its own. You move over the processor, cooler, memory and storage from the old board, and the Windows license stays with the machine.

Is this the right board for my ProDesk 400 G5?

Check the part number on your existing board against L05339-001 or L05339-601. HP used more than one board across ProDesk 400 G5 configurations, so match the number before ordering.

Can I use it to build a new PC?

It works, but it is a poor basis for a build. The proprietary size and connectors restrict cases and power supplies, and a standard LGA1151 board gives more flexibility.