  In this thesis, various extensions of the scalar sector have been considered
comprising of different ${SU(2)_L}$ multiplets. If the extended scalar sector
participates in the electroweak symmetry breaking then these extra scalars need
to couple with the known standard model particles. In this work, it has been
shown that the vector boson scattering involving scalar boson exchanges provide
a complimentary way to direct search methods to probe into the scalar sector.
As different extended scalar sectors have similar types of scalar fields, e.g.,
an extra $CP$-even Higgs, charged Higgs etc., these new physics models can give
rise to the similar types of experimental signatures. It is possible to
distinguish between such models at various vector boson scattering processes by
looking at the resonances. Also, the shapes of the resonances can provide
further insight to the relevant parameter space of these models.
  As in the SM, the electroweak vacuum is metastable, it is important to
explore if an extended scalar has an answer in its reserve. As the scalar
weakly interacting massive scalar particles protected by $Z_2$ symmetry can
serve as viable dark matter candidates, it is interesting to explore if they
help prolong the lifetime of the Universe. The effective Higgs potential gets
modified in the presence of these new extra scalars, improving the stability of
electroweak vacuum. Such an exercise has been undertaken in various kinds of
extended scalar sectors. In order to show the explicit dependence of the
electroweak stability on different parameters of these extended sectors,
various kinds of phase diagrams have been presented. Graphical demonstrations
have been provided to illustrate how the confidence level, at which stability
of electroweak vacuum is excluded, depends on such new physics parameters.
