Dispersion relations: foundations

We give a pedagogical introduction to the founding ideas of dispersion relations in particle physics. Starting from elementary mechanical systems, we show how the physical principle of causality is closely related to the mathematical property of analyticity, and how both are implemented in quantum mechanical scattering theory. We present a personal selection of elementary applications such as the relation between hadronic production amplitudes or form factors to scattering, and the extraction of resonance properties on unphysical Riemann sheets. More advanced topics such as Roy equations for pion--pion scattering and dispersion relations for three-body decays are briefly touched upon.