It is well-known that the class of Functionally Graded Materials (FGMs) has been introduced to reduce some issues, such as delamination and stress peaks, which commonly affect laminated composites or heterogeneous structures characterized by strong discontinuities at the interfaces. By means of a continuous gradual variation of the mechanical properties, which can be defined along a proper path, structures made of FGMs do not show these problems related to the material discontinuities just mentioned. In general, this aim can be reached by mixing two or more constituents, both isotropic and orthotropic, following specified constitutive laws. Consequently, stress peaks, residual stresses, and damage growth can be reduced. The same concepts of FGM are also used to characterize those nanocomposites in which the reinforcing phase is made by Carbon Nanotubes (CNTs). Since their recent discovery, in fact, these constituents have been seen as the perfect candidates to fulfill this aim, due to their excellent properties (thermal, electric, mechanical). The characterization of the properties of CNTs is still an open topic which could be further investigated. Structural elements composed by FGMs or reinforced through CNTs placed according to variable distributions are known as Functionally Graded Material (FGM) and Functionally Graded Carbon Nanotube (FG-CNT) Reinforced Composites, respectively. In general, their mechanical response can be affected by several mechanical parameters, such as the volume fraction distributions of the constituents, their density or agglomeration features, porosity, thermal environment, load and boundary conditions. The Special Issue of Applied Sciences “Functionally Graded Material (FGM) and Functionally Graded Carbon Nanotube (FG-CNT) Reinforced Composites” aims to collect various investigations at different levels (nano-, micro-, and macro-scales), focused on the mechanical analysis of composites reinforced by FGMs and FG-CNTs. Authors are encouraged to present research papers regarding new constitutive models, homogenization methods, advanced applications, as well as numerical, theoretical, and experimental analyses related to this topic, to provide a widespread framework on these innovative materials and facilitate their usage in various engineering fields. Structural problems and analyses are also welcomed.