In this dissertation, I study the many-body physics of two types of collective excitations, excitons and plasmons, in low-dimensional systems.In Chapter 1, I explain how low-dimensional systems are realized and discuss their importance in condensed matter physics. I subsequently introduce excitons and plasmons, as well as exciton-polaritons and exciton-polarons. Chapter 2 considers a novel type of polariton formed by hybridization of excitons in a two-dimensional semiconductor with surface optical phonons or plasmons. We show that these quasiparticles can bind into bipolaritons near a Feshbach-like scattering resonance and analyze the physics of a many-body condensate of polaritons and bipolaritons. In Chapter 3, we study another type of polariton that results from the hybrdiization of magnetoexcitons in graphene with hyperbolic phonon modes in hexagonal boron nitride, and calculate the shift in the magnetoexciton energy due to many-body effects. We investigate excitonic Bose-polarons in Chapter 4, where we develop a many-body theory of these polarons formed by spatially direct excitons immersed in a degenerate Bose gas of spatially indirect excitons. In Chapter 5, we study surface plasmons in minimally-twisted gapped bilayer graphene that develops a triangular network of partial dislocations (or AB-BA domain walls) hosting one-dimensional electronic states, and formulate a theoretical model describing the plasmonic spectrum of the network in different regimes of temperature and electron-electron interaction strength. Conclusions are given in Chapter 6, where I also outline potential future research directions.