This resource makes the full business and ethics case for running a law firm on open-source software, and tests that case against the professional-responsibility duties a lawyer cannot contract away. It explains what open source actually is, how the copyright-based license families (MIT, BSD, Apache, GPL, LGPL, and the SaaS-closing AGPL) differ, and why a firm that merely uses the software owes almost no copyleft obligation. It maps a complete working stack for the operating system, collaboration, documents, communications, accounting, research, and security, then develops a rigorous total-cost-of-ownership comparison with a worked numeric hypothetical. Over all of it sits the ethics overlay: the duty of technology competence under Model Rule 1.1 comment 8, the reasonable-efforts confidentiality duty under Rule 1.6(c) and ABA Formal Opinions 477R and 483, the cloud and supervision opinions, the fifty-state breach-notification patchwork with its encryption safe harbor, and the rising tide of written-information-security-program mandates. It closes with a reversible, phased migration roadmap and a candid inventory of open source's real limits.