  Recent developments in the field of gravitational physics, including the
emergence of gravitational wave astronomy, black hole images, and more accurate
telescopes, have allowed us to probe the strong-field character of gravity in a
novel and revolutionary manner. This accessibility related to strong gravity
brings into the foreground discussions about potential modifications to General
Relativity (GR) that are particularly relevant in high curvature regimes. The
most straightforward way to generalise GR is to consider an additional degree
of freedom, in the form of a scalar field. In this thesis, we study generalised
scalar tensor theories that predict interesting strong-gravity phenomenology.
First, we review scalar no-hair theorems and the conditions under which they
can be evaded. Next, we study solutions of black holes with scalar hair and the
way in which higher derivative terms alter their properties. We then move our
discussion to the spontaneously scalarized solutions, which only deviate from
GR in the strong-field regime. We propose a model consistent with compact
object scalarization, that allows for a GR attractor at late times, without
fine-tuning (EsRGB model). Then, we proceed to study properties of black holes
and neutron stars in this theory, revealing the interesting phenomenology of
the solutions. We also study the radial stability of black holes in EsRGB and
perform a preliminary analysis of the hyperbolicity of the problem. Finally, we
take a look at the shadows of black holes and wormholes in theories with scalar
fields, in light of recent observations of black hole shadows.
