The crystallization step is considered to be a major bottleneck in the process of structure determination by x-ray crystallography. Successful crystallization requires both the formation of nuclei that are capable of supporting crystal growth, and a subsequent crystal growth. Nucleation can occur spontaneously in a supersaturated solution. However, in the commonly used sparse matrix crystallization screens, protein and precipitant concentrations are not extensively sampled, so that suitable supersaturation conditions for nucleation are often missed. This chapter describes a simple method for enhancing nucleation and subsequent crystallization by the addition of heterogeneous nucleating agents.