  The observation of Ising quasiparticles is a signatory feature of the hidden
order phase of URu$_2$Si$_2$. In this paper we discuss its nature and the
strong constraints it places on current theories of the hidden order. In the
hastatic theory such anisotropic quasiparticles are naturally described
described by resonant scattering between half-integer spin conduction electrons
and integer-spin Ising moments. The hybridization that mixes states of
different Kramers parity is spinorial; its role as an symmetry-breaking order
parameter is consistent with optical and tunnelling probes that indicate its
sudden development at the hidden order transition. We discuss the microscopic
origin of hastatic order, identifying it as a fractionalization of three body
bound-states into integer spin fermions and half-integer spin bosons. After
reviewing key features of hastatic order and their broader implications, we
discuss our predictions for experiment and recent measurements. We end with
challenges both for hastatic order and more generally for any theory of the
hidden order state in URu$_2$Si$_2$.
