Insulating oxides grown on silicon have emerged as a major area of research in the past few years. This has been primarily driven by the need to replace silicon dioxide as a gate dielectric in silicon CMOS with a higher dielectric constant material that would provide a higher gate capacitance at a higher physical thickness and therefore a lower leakae current. As is the case in seeking the replacement of a highly successful material such as silicon dioxide within an established processing infrastructure, the new gate oxide material needs to meet a host of parallel requirements related to thermal stability, interfacial stability, and microstructure in order to be a successful product. We will discuss the key problems facing the candidate insulating oxides and present results from molecular beam epitaxially grown aluminum oxide/silicon and rare earth oxide/silicon structures. In addition to the near term technology need for a gate dielectric for CMOS, recent research in insulating oxides on silicon have lead to the development of epitaxial oxide/silicon heterostructures for novel applications. We will discuss the growth and properties of a lanthanum-yttrium oxide that may be grown epitaxially on silicon in lattice matched fashion, allowing us to grow (by MBE) epitaxial oxide-silicon and silicon quantum well based heterostructures.