We study the period in the evolution of the Universe before Big-Bang Nucleosynthesis (BBN). The physics and duration of the period are unknown. We explore hypothetical scenarios involving cosmic inflation and reheating, non-thermal and thermal phase transitions, primordial black holes and cosmic defects production, baryogenesis, and Dark Matter (DM) generation. We use methods of theoretical high-energy physics and cosmology such as non-equilibrium classical and quantum field theory, cosmological perturbation theory and nonlinear general relativity. The main target of our theoretical studies of the pre-BBN scenarios is the calculation of robust experimental signatures such as stochastic Gravitational Wave (GW) backgrounds, features in cosmological correlators, massive relic abundances, potentially observable by upcoming GW, Cosmic Microwave Background and DM detection experiments.