Fold-switching proteins respond to cellular stimuli by remodeling their secondary structures and changing their functions. Whereas several previous reviews have focused on various structural, physical-chemical, and evolutionary aspects of this newly emerging class of proteins, this minireview focuses on how fold switching modulates protein function and regulates biological processes. It first compares and contrasts fold switchers with other known types of proteins. Second, it presents examples of how various proteins can change their functions through fold switching. Third, it demonstrates that fold switchers can regulate biological processes by discussing two proteins, RfaH and KaiB, whose dramatic secondary structure remodeling events directly affect gene expression and a circadian clock, respectively. Finally, this minireview discusses how the field of protein fold switching might advance. Display omitted •Fold-switching proteins remodel their secondary structures in response to stimuli•Six observed varieties of remodeling foster diverse functional changes•Fold-switching proteins regulate gene expression and a circadian clock's periodicity•Several approaches hold promise for the discovery of more fold switchers Fold-switching proteins remodel their secondary structures and change their functions in response to cellular stimuli. Kim and Porter discuss how the dramatic conformational changes in these proteins help to regulate biological processes, such as bacterial gene expression and the periodicity of the cyanobacterial circadian clock.