This unit describes the preparation of conjugates between nanometer-scale gold particles and synthetic oligonucleotides. Oligonucleotide-functionalized gold nanoparticles are finding increased use in both the construction of complex, tailored nanostructures and the optimization of DNA sequence analysis. The protocols in this unit outline the synthesis, purification, and characterization of nanoparticle-DNA conjugates for applications in nanotechnology and biotechnology. Separate procedures are presented for nanoparticles functionalized with just one or a few oligonucleotide strands and for nanoparticles functionalized with a dense layer of oligonucleotide strands. The different physical and chemical properties of these two types of conjugates are discussed, as are their stability and utility in different environments.