These notes provide a comprehensive review of the semiclassical approach for calculating multiparticle production rates for initial states with few particles at very high energies. In this work we concentrate on a scalar field theory with a mass gap. Specifically, we look at a weakly-coupled theory in the high-energy limit, where the number of particles in the final state scales with energy, \(n\sim E\to \infty\), and the coupling \(\lambda\to 0\) with \(n \lambda\) held fixed. In this regime, the semiclasical approach allows us to calculate multiparticle rates non-perturbatively.