The cyclins are a family of proteins that are centrally involved in cell cycle regulation and which are structurally identified by conserved "cyclin box" regions. They are regulatory subunits of holoenzyme cyclin-dependent kinase (CDK) complexes controlling progression through cell cycle checkpoints by phosphorylating and inactivating target substrates. CDK activity is controlled by cyclin abundance and subcellular location and by the activity of two families of inhibitors, the cyclin-dependent kinase inhibitors (CKI). Many hormones and growth factors influence cell growth through signal transduction pathways that modify the activity of the cyclins. Dysregulated cyclin activity in transformed cells contributes to accelerated cell cycle progression and may arise because of dysregulated activity in pathways that control the abundance of a cyclin or because of loss-of-function mutations in inhibitory proteins. Analysis of transformed cells and cells undergoing mitogen-stimulated growth implicate proteins of the NF-κB family in cell cycle regulation, through actions on the CDK/CKI system. The mammalian members of this family are Re1-A (p65), NF-κB1 (p50; p105), NF-κB2 (p52; p100), c-Re1 and Re1-B. These proteins are structurally identified by an amino-terminal region of about 300 amino acids, known as the Re1-homology domain. They exist in cytoplasmic complexes with inhibitory proteins of the IκB family, and translocate to the nucleus to act as transcription factors when activated. NF-κB pathway activation occurs during transformation induced by a number of classical oncogenes, including Bcr/Abl, Ras and Rac, and is necessary for full transforming potential. The avian viral oncogene, v-Re1 is an NF-κB protein. The best explored link between NF-κB activation and cell cycle progression involves cyclin D1, a cyclin which is expressed relatively early in the cell cycle and which is crucial to commitment to DNA synthesis. This review examines the interactions between NF-κB signaling and the CDK/CKI system in cell cycle progression in normal and transformed cells. The growth-promoting actions of NF-κB factors are accompanied, in some instances, by inhibition of cellular differentiation and by inhibition of programmed cell death, which involve related response pathways and which contribute to the overall increase in mass of undifferentiated tissue.