The invention provides the use of silica for cosmetic and/or medical treatment of skin, including improving and enhancing the skin-barrier, anti-aging treatment and photo-protective treatment. The invention is based on the finding that silica exhibits a multi-faceted biological effect useful in the cosmetic and/or therapeutic treatment of skin for the above purposes. Specifically, it has been shown that silica induces markers (involucrin, filaggrin and transglutaminase-1) for skin barrier formation in keratinocytes, induces collagen expression in dermal fibroblasts and inhibits UVA-induced up-regulation of photo-aging markers (matrix metalloproteinase MMP-1 and cytokine IL-6).