Performance requirements in many applications can often be modelled as constraints related to time, for example, the span of data processing for disaster early warning [1], latency in live event broadcasting [2], and jitter during audio/video conferences [3]. These time constraints are often treated either in an “as fast as possible” manner, such as sensitive latencies in high-performance computing or communication tasks, or in a “timeliness” way where tasks have to be finished within a given window in real-time systems, as classified in [4]. To meet the required time constraints, one has to carefully analyse time constraints, engineer and integrate system components, and optimise the scheduling for computing and communication tasks. The development of a time-critical application is thus time-consuming and costly. During the past decades, the infrastructure technologies of computing, storage and networking have made tremendous progress. Besides the capacity and performance of physical devices, the virtualisation technologies offer effective resource management and isolation at different levels, such as Java Virtual Machines at the application level, Dockers at the operating system level, and Virtual Machines at the whole system level. Moreover, the network embedding [5] and software-defined networking [6] provide network-level virtualisation and control that enable a new paradigm of infrastructure, where infrastructure resources can be virtualised, isolated, and dynamically customised based on application needs. The software-defined infrastructures, including Cloud, Fog, Edge, software-defined networking and network function virtualisation, emerge nowadays as new environments for distributed applications with time-critical application requirements, but also face challenges in effectively utilising the advanced infrastructure features in system engineering and dynamic control. This special issue on “time-critical applications and software-defined infrastructures” focuses on practical aspects of the design, development, customisation and performance-oriented operation of such applications for Clouds and other distributed environments.