Over the last decade or so, secondary non-B-DNA structures such as G-quadruplexes and i-motifs have come into focus as biologically functioning moieties that are potentially involved in telomeric interactions and the control of gene expression. In the present short review, we first describe the structural and dynamic parallels with complex RNA structures, including the importance of sequence and ions in folding, and then we describe the biological consequences of the folded structures. We conclude that there are considerable parallels between secondary and tertiary structures in RNA and DNA from both the folding and the biological perspectives.