A core prediction of natural Supersymmetry is the
existence of four light higgsinos not too far above the
mass of the $Z$ boson. The small mass splittings amongst
the higgsinos -- typically 5-20\,GeV --
imply very little visible energy release from decays of heavier
higgsinos.
In particular, if other SUSY particles are quite heavy,
as can be the case in SUSY with radiatively-driven naturalness,
the higgsinos are extremely hard to detect at hadron
colliders. The clean environment of electron-positron
colliders with $\sqrt{s} > 2m_{\mathrm{higgsino}}$, however, would allow for a decisive
search for the required light higgsinos.
Thus, $e^+e^-$ colliders should either discover or exclude natural SUSY.
We present a detailed study of higgsino pair production at
the proposed International Linear $e^+e^-$ Collider which is under
consideration for construction in Japan.
A variety of precision measurements should allow for
extraction of underlying parameters and provide a window onto
physics at the grand unified scale.