  I present some general ideas about quantum entanglement in relativistic
quantum field theory, especially entanglement in the physical vacuum. Here,
entanglement is defined between different single particle states (or modes),
parameterized either by energy-momentum together with internal degrees of
freedom, or by spacetime coordinate together with the component index in the
case of a vector or spinor field. In this approach, the notion of entanglement
between different spacetime points can be established. Some entanglement
properties are obtained as constraints from symmetries, e.g., under Lorentz
transformation, space inversion, time reverse and charge conjugation.
