It is universally accepted that defects in materials scatter thermal phonons, and that this scattering is the reason why defects reduce the flow of heat relative to the defect-free material. However, ab-initio molecular-dynamics simulations which include defect dynamics show that the interactions between thermal phonons and defects involve the coupling between bulk (delocalized) and defect-related (localized) oscillators. Defects introduce Spatially-Localized Modes (SLMs) which trap thermal phonons for dozens to hundreds of periods of oscillation, much longer than the lifetimes of bulk excitations of the same frequency. When a phonon traps in a SLM, momentum is lost and the decay of localized phonons does not depend on the origin of the excitation but on the availability of receiving modes. This strongly suggests that carefully selected interfaces and/or δ- layers can be used to predict and control the flow of heat.