Central to all biological processes are the tightly intertwined concepts of structure-activity relationships and molecular recognition. Traditionally, these concepts were developed for and applied to ordered biomolecules, presenting only one or a few strongly favoured conformations. However, biomolecules lacking an ordered structure have been shown to possess specific biological activity and recognition capability. E.g. proteins with more or less extended intrinsically-disordered regions mediate important signalling processes in eukaryotic cells. In cell signalling, disorder is advantageous because it allows a single protein to interact with multiple binding partners by adopting different structures. Also disordered oligopeptides are able to adapt their conformation to match interfacial features at surfaces of both biological and inorganic matter. Moreover, the activity of antimicrobial peptides results from their induced amphipathic conformation upon interaction with bacterial membranes, A fundamental phenomenon associated with this examples is the so-called 'conformational selection', namely a redistribution (or population shift) of the microscopic states in the biomolecular conformational ensemble as a consequence of the interaction. Such a redistribution of conformational microstates may either destroy or stabilize ordered secondary-structure elements, mediating structure-specific interfacial interactions. The biological significance of such processes cannot be understated, being for instance at the origin of the detrimental activities of amyloid peptides or prions. For this reason, the correct prediction of microstate populations and their interaction-induced redistribution in disordered conformational ensembles is crucial to rationalize and develop strategies against pathologies associated with misfolded and disordered peptides and proteins. Addressing this issue at the molecular level poses great challenges both to experimental and theoretical approaches. Easily accessible experimental observables, such as lightabsorption spectra of dissolved oligopeptides, are averaged over all ensemble microstates (conformers), each occurring with a probability specified by Boltzmann statistics. The issue here is that the free-energy landscapes in the conformational phase space of disordered peptides and proteins are wide and shallow, resulting in a very high number of conformers contributing almost equally to the ensemble average. Due to this, even the most advanced structure prediction methods, including AlphaFold, are powerless in exploring such a conformational ensemble and therefore the "protein folding problem" remains unsolved for preferentially disordered systems. Furthermore, theoretical methods capable of computing from first principles the spectral response of molecules are very demanding, limiting the number of conformers that can be investigated. As a consequence, interpretation of the behaviour of disordered peptide and protein systems in terms of structure-function relationships, or the molecular-scale elucidation of their biological recognition processes, requires a paradigm shift in the way experimental observables are predicted from atomistic simulations.