One notable line of inquiry, aspects of which date back to the early 1980s, involves the design and implementation not of long-term curricula, but of topic-oriented sequences for teaching science. One distinguishing characteristic of a teaching-learning sequence (TLS) is its inclusion in a gradual research-based volutionary process aiming at interlacing the scientific and the student perspective. In the present paper, which is introductory to the special issue, we attempt to serve a double purpose: on the one hand, we provide an overview of developments and trends with regard to TLSs and their classroom validation, discussing empirical studies, theoretical proposals, methodological tools and approaches to describing the design of these sequences in ordinary language, while on the other the paper serves as an introduction to this volume, making it easier for the reader to apprehend the processes of development and validation of research on TLSs.