Distributed quantum computing (DQC) connects quantum processing units (QPUs) to execute programs beyond the capacity of a single processor. Understanding its scalability requires examining the interactions between hardware, communication, software, and security. This structured review synthesizes 126 studies, mainly published between 2020 and 2026 with foundational earlier work, retrieved from eight sources and selected using documented inclusion and exclusion criteria. We organize the literature into five pillars: infrastructure, protocols, applications, compilation, and security. A common scoping rule distinguishes work enabling distributed computation from broader quantum communication research. The synthesis identifies interconnect rate and fidelity, classical control latency, remote-operation overhead, and coherence constraints as coupled limits on distributed execution. Infrastructure and protocol research includes experimental demonstrations, whereas application and compiler evaluations remain predominantly simulation-based. Security requires both protected interconnects and confidential, verifiable delegated computation. Comparisons across the pillars reveal tradeoffs between local computation, communication resources, sampling overhead, and trust assumptions. The review highlights unresolved challenges in distributed fault tolerance, transduction, interoperable software, benchmarking, and integrated security. Its central conclusion is that scalable DQC requires coordinated hardware–software–network design and evaluation of complete systems rather than isolated improvements within individual layers.