The **Native API** is a lightweight application programming interface (**API**) used by [[Windows NT]] and user mode applications. This **API** is used in the early stages of [[Windows NT]] startup process, when other components and **APIs** are still unavailable. Therefore, a few Windows components, such as the Client/Server Runtime Subsystem ([[CSRSS]]), are implemented using the **Native API**. The **Native API** is also used by subroutines such as those in [[kernel32.dll]] that implement the [[Windows API]], the **API** based on which most of the Windows components are created.

Most of the **Native API** calls are implemented in [[ntoskrnl.exe]] and are exposed to user mode by [[ntdll.dll]]. The entry point of [[ntdll.dll]] is **LdrInitializeThunk**. **Native API** calls are handled by the kernel via the System Service Descriptor Table ([[SSDT]]).