RNA nanotechnology seeks to exploit the structural and functional properties of the RNA molecule in order to rationally design RNA nanoparticles and devices for applications in biotechnology and medicine, among others. Compared to DNA, RNA can adopt a larger diversity of structural motifs that allows the construction of more complicated nanoparticles that can be self-assembled during synthesis by the RNA polymerase—a process called co-transcriptional folding. RNA nanostructures can be genetically encoded and co-transcriptionally folded in cells, which allows large-scale production of RNA nanoparticles for therapeutic use or the application as scaffolds in cells for manipulating cellular components for use in synthetic biology. In this article, we describe the origins of the RNA nanotechnology research field and how it has been inspired by DNA nanotechnology. Recent developments of co-transcriptionally folded RNA nanostructures and the construction of RNA knots are discussed in relation to design principles and challenges, and speculations about future directions of the field are provided.