Galactic Law was the system of law, similar to international law on Earth, held by many interstellar governments in Mutter's Spiral, and is enforced by various species, including the Judoon (DW: Smith and Jones) and the Megara. (DW: The Stones of Blood) The Galactic Alliance were keen upholders of Galactic Law. (SJA: Funny for Money)
Important documents of Galactic Law include the Shadow Proclamation (DW: Partners in Crime) and the Galactic Charter. (DW: The Stones of Blood)
Galactic Law said that developing worlds classed as Level 5 planets (such as Earth in the 21st century and earlier) could not be approached for legal matters, so they had no jurisdiction over the planet, and methods such as a H2O Scoop must be used to transport any places required to be visited to neutral territory, such as the Moon. (DW: Smith and Jones)
Seeding life on a Level 5 planet is also against Galactic Law. (DW: Partners in Crime)
Galactic bi-law 19b dictated that dogs must be kept on leads. (SJA: Funny for Money)