Black hole scalarization
General Relativity remains our best description of gravitational phenomena. Nonetheless, issues such as its quantization and the cosmological constant problem suggest that Einstein's theory might not be the final theory of gravitational interaction. Motivated by these questions, theorists have proposed many extensions to General Relativity over the decades. In this presentation, I will focus on theories with extra scalar fields. In particular, I will describe how some of these theories can evade Solar System constraints and yield new effects in the strong-gravity regime of compact objects, i.e., neutron stars and black holes. This is achieved through a process known as spontaneous scalarization, in which the compact object grows "scalar hair" once certain conditions are met and remains "bald" otherwise. I will review the basics of this effect and then give a short bird's-eye view of our understanding of scalarization for black holes both in isolation and binaries.