The nuclear factor (NF)-kappaB family of transcription factors serves vital roles in a wide array of cell functions. An increasing number of human genetic lesions that result in defined disease entities are linked to inappropriate activation of NF-kappaB. The resulting aberrant NF-kappaB function can lead to cellular defects that ultimately impair normal developmental processes, host immune defenses, or both. Molecular defects that lie upstream in cell-signaling pathways and rely upon NF-kappaB activation tend to give a more specific phenotype, whereas those closer to the actual NF-kappaB proteins have broader defects. A detailed study of these diseases can provide insight into the biochemistry of NF-kappaB activation as well as the role of NF-kappaB in human health.