  For itinerant fermionic and bosonic systems, we study `particle
entanglement', defined as the entanglement between two subsets of particles
making up the system. We formulate the general structure of particle
entanglement in many-fermion ground states, analogous to the `area law' for the
more usually studied entanglement between spatial regions. Basic properties of
particle entanglement are uncovered by considering relatively simple itinerant
models.
