Many machine parts become unusable as a result of wear and corrosion on their surfaces, depending on their environment and operating conditions. Changing machine parts causes both production disruption and financial damage. Therefore, increasing the lifespan of machine parts is of great importance. For this purpose, instead of producing machine parts from a high-cost material, the surface of the parts is coated with a material with better properties or a protective layer is created on the surface of the parts by thermochemical processes. These layers formed on the surface of the parts protect the substrate material from conditions such as wear and corrosion. Carbide, nitride and borided structures are formed on the part surface, respectively, by thermochemical processes such as carburizing, nitriding and boriding. These structures have high hardness values and show superior wear and corrosion resistance. Thermochemical processes, especially carburizing and boriding, are generally carried out in solid media for reasons such as being simple and economical. In processes carried out in solid media, powder environment containing carbon, nitrogen or boron sources and various activators to accelerate the  reaction are used, depending on the type of process performed. Rare earth elements has an increasing effect on the diffusion of carbon, nitrogen or boron in thermochemical carburizing, nitriding and boriding processes, respectively. In this study, the effect of rare earth element additives in the boriding process on the properties of the formed boride layer was investigated. When we examine the studies in the literature, it is seen that even small amounts of additives have effects on the properties such as microstructure, thickness and hardness value of the formed boride layer. It has been observed that ironbased materials, titanium alloys and rarely nickel alloys are used as substrate materials. Although it is seen that lanthanum and cerium element oxides are generally used as rare earth element additives, there are also studies using neodymium, gadolinium, ytterbium, yttrium and samarium compounds. It has been observed that oxidized compounds of rare earth elements are more preferred than nitrate and chloride compounds as additives. In general, it can be said that there are a limited number of studies on the addition of rare earth element compounds to boronizing media. In this context, it has been concluded that the use of different substrate materials and rare earth element additives in the boriding process will make a great contribution to the literature.