The recognition of cyclin-dependent kinase (CDK)/cyclin complexes by various cell-cycle regulatory proteins, including certain tumour suppressors and transcription factors, occurs at least in part through a protein-protein interaction with a binding groove on the cyclin subunit. Since CDK function is generally deregulated in tumour cells, blocking of this recruitment site prevents recognition and subsequent phosphorylation of CDK substrates and offers a therapeutic approach towards restoration of checkpoint control in transformed cells. Here we discuss the finding that peptides derived from such cyclin-interacting proteins, and rendered permeable through conjugation to cellular delivery vectors, can apparently induce tumour cells to undergo apoptosis selectively. We review the current status of 3D-structural information available on cyclin-peptide interactions and we summarise our extensive peptide structure-activity relationship studies in light of this information. We also show how a combination of molecular modelling and introduction into synthetic peptides of peptidomimetic elements, such as non-natural amino acid residues and conformational constraints, is being used hopefully to arrive at drug candidates capable of modulating CDK function in a selective mechanism-based approach rather than through ATP antagonism.