The Constitution allows the government to "take" private property if it is needed in order to complete certain types of public projects. They do so through formal condemnation proceedings. Projects that can result in this type of action are widening of public roads/freeways, the building of public transportation systems, etc. This "right to take" is established by the 5th Amendment and Article 1, Section 19 of the California Constitution. The official term for this type of action on the part of a government agency is eminent domain.