## 1 Introduction  
Electrodeposition and electroless deposition are among the most commonly used methods for fabricating metallic or composite coatings, porous layers, or nanostructures [1]. These solution-based techniques enable the controlled and scalable synthesis of single- or multicomponent materials under mild processing conditions, at temperatures below 100 \({}^{\circ}\)C and under atmospheric pressure, and without the need for complex or expensive equipment. Adjusting the process parameters and bath components allows for the composition, thickness, and morphology of the deposited layers to be modified, thus achieving structural arrangements and properties that are often unattainable by other manners [2]. Electrochemical procedures offer high deposition rates and fine control via the application of current or potential, using aqueous or non-aqueous electrolytes containing metal salts. Electroless deposition, on the other hand, enables uniform coating from aqueous solutions on complex-shaped substrates without the need for an external power source. Owing to these benefits, both techniques have been recently increasingly explored for the synthesis of polymetallic alloys, including high-entropy alloys (HEAs) [3, 4] and medium-entropy alloys (MEAs) [5]. The concept of high-entropy alloys was introduced about twenty years ago [6]. It involved a novel approach to alloy design, based on combining multiple principal elementsin equimolar or near-equimolar ratios, resulting in high configurational entropy (above 1.5R, where R is the gas constant), which lowers the free energy of the structure (Figure 1). This contrasts with the traditional alloy model, where one or two principal metals are combined with smaller amounts of alloying elements, and the formation of brittle intermetallic compounds is common when incorporating a wide variety of elements, whether metallic or nonmetallic. High-entropy alloys are typically solid solutions with FCC or BCC structures, composed of at least five principal elements, each present in concentrations ranging from 5 to 35 at.%. However, second-generation HEAs may consist of at least four principal elements and form dual or complex phases in non-equimolar compositions [7]. Their high configurational entropy promotes the formation of simple phases, while sluggish diffusion slows down microstructural processes, and severe lattice distortion enhances both the chemical and mechanical properties [6; 7]. The complex interactions among multiple elements give rise to the so-called \"cocktail effect\", showing behavior that exceeds predictions based on the rule of mixtures. As a consequence, high-entropy alloys exhibit superior mechanical strength [8], corrosion resistance [9], magnetic properties [10], and catalytic activity [11; 12] compared to conventional alloys. These advantages, together with virtually unlimited compositional flexibility, create new opportunities for the development of advanced functional materials for different applications through various synthesis routes [8; 11; 12; 13; 14; 15]. While the first electrodeposited high-entropy alloys were reported in 2008 [16] and have since been explored in various multimetallic systems [3; 4], the application of electro-less plating has remained considerably more challenging, with the first successful attempts published in 2020 [17]. In light of these recent developments, this review aims to highlight the growing interest in the use of electrochemical methods for HEA synthesis, covering both electrodeposition and electroless deposition approaches. Particular emphasis is placed on the role of electrolyte composition and current/potential modes in tailoring the structure and morphology of HEA coatings, which ultimately determine their properties and technological potential. The latter is especially important, as thin films of HEAs can exhibit not only high-performance properties comparable to those of bulk high-entropy alloys but may even surpass them, providing a significant improvement to the surface properties of the substrate [15].