The twin-supercoiled-domain model describes how transcription can drive DNA supercoiling, and how DNA supercoiling, in turn, plays an important role in regulating gene transcription. In vivo and in vitro experiments have disclosed many details of the complex interactions in this relationship, and, recently, new insights have been gained with the help of genome-wide DNA supercoiling mapping techniques and single-molecule methods. This review summarizes the general mechanisms of the interplay between DNA supercoiling and transcription, considers the biological implications, and focuses on recent important discoveries and technical advances in this field. We highlight the significant impact of DNA supercoiling in transcription, but also more broadly in all processes operating on DNA. © 2016, International Union for Pure and Applied Biophysics (IUPAB) and Springer-Verlag Berlin Heidelberg.