As its title implies, this work examines forms of linguistic intervention available to the public powers, and lists and comments the legal techniques that exist to protect languages and linguistic groups in multilingual societies. A brief introduction (section 1) recaps on the reasons for the increase in the number of societies with languages in contact, and warns us that these languages in contact do not generally share a position of equilibrium. Subsequently (section 2) the work underlines the need to distinguish between public or official relationships and those of a private nature (inter privatos) when analysing linguistic interventions and studying legal techniques for their protection. Further to this question of private relationships, mention is made of the convenience of distinguishing between relationships that take place in public places or where there is an affluence of public (for example advertising in the public highway) and others of a strictly private or family nature, or which pertain to individuals' personality rights (for example the language spoken at home, the name of individuals). These distinctions are relevant, because restrictive, constrictive or imposed linguistic interventions cannot be the same in the three cases mentioned: while such interventions are common in the public or official sphere, in the private scenario they are subject to major limitations, but are always illicit in the strictly private or family setting. This is followed (section 3) by an exposition of the different finalities and objectives that may be pursued by linguistic interventions, whose extreme and conflicting manifestations would be the assimilation and disappearance of a language or on the other hand the promotion, protection and harmonisation thereof. The work goes on to address the issue of assimilationist linguistic interventions, and the reasons or causes behind them, which may be political, religious or politico-economic. The justifications that tend to come with such assimilationist interventions, ostensibly "hygienic" or "altruistic", are also dealt with. These assimilationist interventions are set against non-intervention approaches, whose non-neutral nature is highlighted, as well as protective interventions, which, along with the legal techniques they give rise to, are the object of the considerations and reflections of the remaining sections of this work. Following a summary exposition (section 4) of the two major principles that underpin linguistic policies, and therefore linguistic interventions (the principles of territoriality and personality), the work analyses the consequences arising from a language being classified as official (section 5). These importance of these consequences, mentioned and developed in the work, make it possible to assert that official recognition is probably the most incisive legal technique for protecting a language. Thus, the fact that a language does or does not reach the status of official language is of major import. The work proceeds with a detailed list, based on Comparative Law, of the main legal techniques that exist and are used to protect and safeguard languages and linguistic groups (section 6). The numerous techniques detailed are systematised under three headings depending on whether they pertain to legal protection techniques applicable to private relationships (and here the distinction is made between non-constrictive and constrictive techniques), legal protection techniques applicable to education or techniques applicable to public or official relationships. Although it is not a specific and direct linguistic intervention, since this is something that is outside the sphere of these interventions, the following section addresses the linguistic content of fundamental rights (section 7). In so far as the contents of some of these rights have a linguistic dimension, this inherent linguistic content must be respected if the corresponding basic right is not to be violated. It is therefore important to highlight the linguistic dimension of basic rights, since in multilingual States where there is a charter of basic rightsand freedoms, but it transpires that linguistic groups are systematically ignored, this dimension will make it possible to obtain from the courts at least the respect for the linguistic rights inherent in the basic rights. In the protection of languages and linguistic groups neither should the importance of the function played by international treaties be underestimated, an aspect which is addressed in section 8. The work concludes with some closing considerations (section 9). The warning that Law, regarded in isolation, and without underplaying its importance and impact, is insufficient as an instrument for the promotion and protection of languages, is underscored. There are numerous contexts where suitable legal measures for a really effective protection of languages must go hand in hand with a veritable social transformation. In any event, the work concludes that multilingual states, if they really aspire to be plural and democratic, must be firmly committed to promoting and safeguarding the languages that are traditionally and historically spoken in them.