  The notion of Wigner particles is attached to irreducible unitary
representations of the Poincare group, characterized by parameters m and s of
mass and spin, respectively. However, the Lorentz symmetry is broken in
theories with long-range interactions, rendering this approach inapplicable
(infraparticle problem). A unified treatment of both particles and
infraparticles via the concept of particle weights can be given within the
framework of Local Quantum Physics. They arise as temporal limits of physical
states in the vacuum sector and describe the asymptotic particle content. In
this paper their definition and characteristic properties are worked out in
detail. The existence of the temporal limits is established by use of suitably
defined seminorms which are also essential in proving the characteristic
features of particle weights.
