We have imaged for the first time the atomic structure of a quasicrystal using a scanning tunneling microscope. The structure of the decagonal AI65C020CU15 quasicrystal is described by a pentagoncenter-decorated pentagonal quasilattice giving strong evidence for a tiling model. The scanningtunneling-microscope measurements of the surface are in good agreement with the high-resolution xray-diffraction measurements of the bulk.PACS numbers: 61.50. Em, 61.55.Hg, 63.90.+t