  These notes comprise a part of the introductory lectures on Higher Spin
Theory presented in the Eighth Modave Summer School in Mathematical Physics. We
construct free higher-spin theories and turn on interactions to find that
inconsistencies show up in general. Interacting massless fields in flat space
are in tension with gauge invariance and this leads to various no-go theorems.
While massive fields exhibit superluminal propagation, appropriate non-minimal
terms may cure such pathologies as they do in String Theory--a fact that we
demonstrate. Given that any interacting massive higher-spin particle is
described by an effective field theory, we compute a model independent upper
bound on the ultraviolet cutoff in the case of electromagnetic coupling in flat
space and discuss its implications. Finally, we consider various possibilities
of evading the no-go theorems for massless fields, among which Vasiliev's
higher-spin gauge theory is one. We employ the BRST-antifield method for a
simple but non-trivial gauge system in flat space to find a non-abelian cubic
coupling and to explore its higher-order consistency.
