This paper explores key aspects and computer security challenges to digital instrumentation and control systems, resulting from new and innovative designs of small modular reactors and microreactors, and which have an impact on their deployment and operation. In the last two decades, the interest in development and deployment of small modular reactors and microreactors has increased due to many driving forces, but of primary interest for computer security are those associated with the innovative attributes taking advantage of advances in digital technologies. Such attributes include, digital instrumentation and control, increased digital automation, and remote monitoring and supervisory control that may lead to improvements in the overall plant capacity factor; the capability of increasing installed capacity within a relative short time; and the lower capital cost compared to traditional nuclear power plants among others. Such features reinforce the need for instrumentation and control solutions and computer security measures being considered and maintained during the entire small modular reactors and microreactors lifecycle, from design to operation to decommissioning. Innovative small modular reactors and microreactors attributes may introduce unintended vulnerabilities to cyber-attack, for example, the need for secure communications between the local control room and remote operation and maintenance centers; the use of mutualized plant systems with digital technologies new to the nuclear industry, e.g. smart sensors and inferred process measurements; and strategies for modular construction with a more complex digital supply chain. The IAEA has been discussing with the international nuclear community these key computer security aspects and challenges related to the application of digital instrumentation and control and emerging digital technologies in the design and deployment of small modular reactors and microreactors. As participants in these discussions, in this paper, we highlight some of the key themes that have emerged from these discussions.