In this work effect of trace additions of Sr, P and Fe on heterogeneous nucleation of Si was investigated in high purity eutectic Al-Si and Al-Mg₂Si alloys. An entrained droplet technique was adapted to study the heterogeneous nucleation in Al-Si alloys. This technique was proved to be useful due to its high reproducibility. The Al-Si alloys were prepared in an arc melter under Ar atmosphere. The alloys were melt-spun to subsequently investigate the nucleation events in small entrained droplets using DSC, SEM, TEM and HRTEM. The controlled additions of Sr in Al-Si alloys revealed that Sr exert no positive effect on the nucleation process rather increased undercooling, was observed. This can be attributed to the formation of Sr3P2 having lower and preferentially formed as compare to other competing phosphide compounds. The estimated patch size of AlP was found to be sufficient for the free growth of Si to occur inside the droplet. Al2Si2Sr intermetallics did not found to contribute in the nucleation of Si. No evidence of nucleation of Si on β-AlSiFe phase was observed and Fe phases were found to be a poor catalytic nucleation site for Si while only traces of P have profound effect on nucleation. Furthermore, high undercooling, and Sr additions mainly control the twin density in Si crystals. The concentrated Sr at the twin re-entrant corner was detected for the first time. In Al-13.1 wt. % Mg2Si alloys, selected from eutectic channel position, the effect of cooling rate, P and Te additions on eutectic microstructure was studied. The Al-Mg2Si eutectic alloys without and with controlled additions of P and Te were manufactured by conventional melting and casting. Alloys were cast in permanent especially designed cast iron mould, a provision for filter to limit the effect of bifilms. The mould temperature was varied to investigate the effect of cooling rate. It was found that the cooling rate drastically refines the microstructure in hypoeutectic Al-Mg2Si alloys, whilst P additions exert no effect on the eutectic spacing up to a critical level and refined eutectic cells were observed. A high P content suppressed the precipitation of ternary eutectic. This effect of P never reported in the literature. Te additions in high purity Al-Mg2Si alloy were found to have no significant effect on the eutectic microstructure. The present work demonstrates the strong influence of trace elements such as P on the nucleation of intermetallics and Si in Al alloys.