There is no better introduction to the principles and operation of RSFQ (Rapid Single Flux Quantum) logic than the seminal review of Likharev and Semenov [1]. It is impossible, however, to encompass in a single paper all the aspects of this rapidly changing and expanding field, so for the latest information and a broader outlook the reader is referred to the more recent reviews [2, 3]. It is not my goal here to cover the history of superconductive digital electronics and to compare the (clearly winning, in my view) RSFQ approach to various voltage-state logics and other SFQ-based schemes, although their achievements are certainly worthy of note (for less biased and more broadly representative overview, see, e.g., [4]). Many interesting on-going RSFQ projects will also not be covered, instead, the focus will be on what is essential for further discussion.