Substellar companions, also known as brown dwarfs, are celestial objects that are intermediate in size between planets and stars. They are not able to sustain nuclear fusion in their cores like stars, but they are too massive to be considered planets. Substellar companions are typically found orbiting stars, either as companions in binary systems or as lone objects. These objects are important for understanding the formation and evolution of planetary systems and can help astronomers better understand the boundaries between planets and stars. Research in this area focuses on studying the properties, formation mechanisms, and demographics of substellar companions in order to gain insights into the diversity of planetary systems in the universe.