  Massive and massless potentials play an essential role in the perturbative
formulation of particle interactions. Many difficulties arise due to the
indefinite metric in gauge theoretic approaches, or the increase with the spin
of the UV dimension of massive potentials. All these problems can be evaded in
one stroke: modify the potentials by suitable terms that leave unchanged the
field strengths, but are not polynomial in the momenta. This feature implies a
weaker localization property: the potentials are "string-localized". In this
setting, several old issues can be solved directly in the physical Hilbert
space of the respective particles: We can control the separation of helicities
in the massless limit of higher spin fields and conversely we recover massive
potentials with 2s+1 degrees of freedom by a smooth deformation of the massless
potentials ("fattening"). We construct stress-energy tensors for massless
fields of any helicity (thus evading the Weinberg-Witten theorem). We arrive at
a simple understanding of the van Dam-Veltman-Zakharov discontinuity
concerning, e.g., the distinction between a massless or a very light graviton.
Finally, the use of string-localized fields opens new perspectives for
interacting quantum field theories with, e.g., vector bosons or gravitons.
