  The apparent absence of new heavy states at the LHC below the TeV scale
points that there is a gap in the electroweak energy spectrum. This scenario is
conveniently described through the electroweak effective theory, an effective
description that contains the Standard Model fields and symmetries. A generic
non-linear realization of the electroweak symmetry breaking pattern $SU(2)_L
\otimes SU(2)_R \to SU(2)_{L+R}$ with a singlet Higgs is adopted, with no
assumption about how it couples to the Standard Model. To seek for heavy
particles, we formulate the electroweak resonance theory as the most general
high-energy Lagrangian including both fermionic and $J^P = 0^\pm,\, 1^\pm$
bosonic heavy states, where all the resonances can form n-plets under the
electroweak and the color group. An extended analysis of the imprints that the
heavy states leave on the low-energy theory is performed, with an special
emphasis on spin-1 fields and their formalism implementation. We study the
phenomenology of the $P$ even purely bosonic sector of the resonance theory,
where the low-energy predictions are refined with the assumption of very
reasonable ultraviolet constraints.
