We describe recent topics of rapid-single-flux-quantum (RSFQ) circuits. Much higher integration and higher-speed operation are required for commercializing the RSFQ circuits in any application. To satisfy this requirement, we measure several kinds of time fluctuations that possibly limit the integration level and operating speed, and introduce the flexible passive transmission line technology including a multicasting transmitter. In addition, we describe our ongoing applications. A multiple-detectors-system is expected to open a new application field of an RSFQ-based analog-to-digital converter and a digital signal processor. As for a microprocessor application, increased number of arithmetic logic units may be effective for enhancing the computing performances.