Selective oxidation of AlAs (or AlGaAs) can be used to form buried, low refractive index apertures within high Q Fabry-Perot microcavities. These apertures provide electrical and optical confinement, and for vertical-cavity surface-emitting lasers (VCSELs) have resulted in ultra-low threshold room temperature lasing with threshold currents under 25 μA. When used with quantum dot light emitters, the oxide-apertured microcavity can also be used to control the spontaneous lifetime. We describe the microcavity fabrication based on high Q Fabry-Perot microcavities and selective oxidation, and design and cavity Q constraints for apertured microcavities for quantum well and quantum dot VCSELs and microcavity LEDs. Threshold current densities of quantum well VCSELs are as low as 98 A/cm2, while ground state lasing is also obtained for quantum dot VCSELs. Our initial experiments on microcavities with very small apertures and quantum dot emitters demonstrate up to a factor of 2.3 increase in the spontaneous emission rate.