Explore essential cosmological perturbation theory in this comprehensive lecture by David Wands from the International Centre for Theoretical Sciences. Delve into the cosmological standard model, probes of cosmological perturbations, and the composition of the cosmos today. Examine Friedmann's dynamic cosmology and waves in an expanding universe. Investigate metric perturbations, including gravitational waves and vector perturbations. Analyze linear gauge transformation rules and their implications for gauge dependence. Study linear perturbation dynamics, adiabatic perturbations, and the ζN formalism for primordial density fluctuations. Discover models for the origin of cosmic structure, including passive models and single-field scenarios. Gain insights into higher-order correlators and their significance in cosmology.