Al-Mg-Si alloys are the most frequently used group of age hardenable aluminum alloys. The adverse effect of natural pre-aging on the artificial aging, a problem of significant academic as well as industrial interest, has not been fully resolved since it was discovered in 1939. In this thesis a new concept for understanding this effect is presented. Microstructural observations and an analysis of precipitation and dissolution kinetics revealed a dependence on the artificial aging temperature for the effect of natural pre-aging. This was explained by introducing the “vacancy-prison mechanism.” The model assumes that Mg,Si co-clusters, formed during natural pre-aging, reduce the mobility of quenched-in vacancies during artificial aging. The temperature dependent stability of these clusters induces the adverse effect of natural pre-aging at industrial common artificial aging temperatures but enhances artificial aging at high temperatures. A comparative approach of atom probe tomography and transmission electron microscopy revealed that the important β´´ phase contains Al and shows a Mg/Si ratio higher than expected from the previously reported Mg5Si6 stoichiometry in the alloy AA6061. Its size distribution was observed to strongly depend on the thermal history and its genesis was successfully explained by the “vacancy-prison mechanism.” The nucleation of the β´´ phase was studied via a multi-method approach using atom probe tomography, transmission electron microscopy, electrical resistivity and hardness measurements, and differential scanning calorimetry. It was shown that quenched-in vacancies are of particular importance for the nucleation of β´´, an important reason for the adverse effect of natural pre-aging. The influence of the alloy composition was addressed by investigating the dissolution kinetics of Mg,Si co-clusters in different alloys. Theoretical considerations concerning the annihilation of quenched-in vacancies during artificial aging allowed extending the “vacancy-prison mechanism” to the whole group of Al-Mg-Si alloys. Moreover, it was shown that the dependence of precipitation kinetics on the solute super-saturation is a function of the thermal history in Al-Mg-Si alloys. This is an exception from common diffusion controlled precipitation kinetics, but can be well explained applying the “vacancy-prison mechanism.” The presented new concept for understanding the effect of natural pre-aging on the artificial aging of Al-Mg-Si alloys describes the observed phenomenon quite well in a wide temperature and compositional range. It will significantly contribute to many industrial issues. Based on the fundamental results, a modified industrial quenching procedure was already presented as a new heat treatment strategy to avoid the negative effect of natural pre-aging without requiring an additional pre-aging step. Furthermore, it was shown that the effect of natural pre-aging can be influenced by trace additions of “vacancy active” elements, making a 70 year old problem controllable.