HATs
Hardware Attached on Top: add on boards that connect to a Raspberry Pi to extend its capabilities with additional compute, storage, or interfaces.
HAT stands for Hardware Attached on Top. It is the Raspberry Pi Foundation's specification for add-on boards that extend what a Raspberry Pi can do.
What they add
HATs connect to the Pi via its GPIO header or, on the Raspberry Pi 5, via the onboard PCIe FPC connector. They can add AI acceleration, NVMe storage, additional ports, sensor interfaces, and more. The Raspberry Pi AI HAT+ 2 is one example, adding a Hailo neural network accelerator for local AI inference. M.2 NVMe HATs are another common type, providing fast solid state storage via the PCIe interface.
The PCIe constraint
The Raspberry Pi 5 has a single PCIe interface, accessible via an FPC connector on the board. HATs that use this interface, including AI HATs and NVMe HATs, compete for the same connection. Only one can be used at a time unless a specific multi interface board is used. This is worth considering before purchasing multiple HATs intended for the same Pi.
Fitting and cooling
Some HATs are not compatible with certain Pi cases, particularly cases that enclose the GPIO header area. If you are planning to add HATs later, factor that into your case choice from the start. Removing and reapplying thermal compound is achievable but adds a step to what should otherwise be a quick upgrade.