After demonstrating that both Amdahl's law and the Universal Scalability law (USL) are fundamental, in that they can be derived from queueing theory, the USL is used to quantify the scalability of a wide variety of distributed systems, including Memcache, Hadoop, Zookeeper, AWS cloud, and Avalanche DLT. Contact is also made with some well-known concepts in modern distributed systems, viz., Hasse diagrams, the CAP theorem, eventual consistency and Paxos coordination.