The material properties of nanometer-sized clusters are dependent on the cluster size. Changing the cluster dimensions induces structural phase transformations, metal-insulator transitions, non-linear optical properties and widening of the band gap of semiconductors. In this work, nanoclusters are created by ion implantation followed by thermal annealing. The ceramic oxides MgO and Al2O3 are used as embedding materials because of their stability and optical transparency. All clusters were created by means of ion implantation (typical dose 1016 ions cm-2, energy range 0.031.0 MeV) and subsequent thermal annealing (temperature 9001300K). Many experimental techniques were applied to investigate the defect evolution and the properties of the nanoclusters, amongst others optical absorption spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and positron annihilation spectroscopy. The formation, growth and dissociation of He and Kr bubbles in MgO and Al2O3 was investigated. It is found that the Kr nanoclusters in MgO are solid and have the fcc crystal structure. The lattice parameter of the Kr in individual clusters was found to vary as a function of the cluster size. Metallic Li, Zn, Ag and Au nanoclusters in MgO are also studied. XRD and TEM investigations reveal that Ag and Au nanoclusters are in a cube-on-cube orientation relationship with the rocksalt MgO. Large Li clusters (> 40nm) have the bcc crystal structure, while small Li clusters (