Magnesium hydroxide and magnesium oxide are compounds with favourable and unique properties, leading to a broad range of opportunities for their use in science, and above all in practical applications. This review describes methods of synthesising both Mg(OH)2 and MgO, including precipitation, solvothermal and hydrothermal processes, the sonochemical technique, the sol-gel, electrochemical and microwave methods, and paths based on calcination of magnesite, carbonatisation, and chemical vapour deposition. This comprehensive literature study provides extensive information on the properties and applications of these compounds and methods by which they may be obtained and modified. Reference is made to possibilities of using magnesium hydroxide as, among others, an antibacterial agent, a neutraliser of pollutants in fresh and wastewater, and a component of membranes, but the greatest detail is given in relation to the use of Mg(OH)2 as a flame retardant. Magnesium oxide, in turn, is characterised as an antibacterial agent and as a fire-resistant and insulating material. Particular attention is given to questions relating to the use of MgO as an adsorbent, as an active catalyst, and as a catalyst support. The latest developments in methods of modification of magnesium hydroxide and magnesium oxide, divided into those carried out using organic compounds and those using inorganic compounds and metals, are also presented. A critical assessment of the current state of knowledge is presented, as well as indications of new directions for further research into the development of magnesium hydroxide and magnesium hydroxide. This review is a source of scientific as well as practical information on the possible applications of Mg(OH)2 and MgO at the present time and methods of obtaining them, in the light of available and applied research techniques and discoveries made to date in the field of advanced powder technology. Display omitted •Synthesis of multifunctional magnesium hydroxide and oxide by different method•Modification methods of Mg(OH)2 and MgO using organic and inorganic compounds•Possible paths of application of Mg(OH)2 and MgO