Planar printed antenna technologies, due to their light weight, low profile, cost effectiveness and ease of connection with the active devices (e.g., amplifiers etc.), are becoming an attractive solution for the commercial data-hungry applications, such as 60 GHz high-data rate communication. Another commercial application of planar antennas includes the exponentially growing lucrative industry of automotive radars. Such radar systems not only require highly reliability since human lives are involved, but also extreme integration to serve the purpose of compactness and cost. Therefore, it is desirable to have all the system functionalities on a single silicon chip, which can then easily be combined with either off- or on-chip antennas/antenna arrays. In such scenarios, planar printed antennas/arrays, because of their low profile and ability to be integrated, are emerging as an alternative to the bulky and expensive dielectric lenses. Although planar antennas show advantageous properties, there are two major challenges associated with their design, namely surface waves and front-to-back radiation ratio. In this thesis, a innovative planar methodology is presented to solve the aforementioned bottlenecks. The proposed technique can be used to obtain simultaneously high radiation efficiency (i.e., minimal surface wave excitation) and good front-to-back radiation ratio. This solution consist of engineering anisotropic equivalent materials, referred to as artificial dielectric layers (ADLs), and using them to enhance the performance of planar antennas. A practical planar realization of this concept can be achieved by embedding inside the host dielectric a periodic array of sub-wavelength square metal patches in a multilayer configuration. In this work, the main aspects pertaining to the theoretical development and the practical implementation of ADLs are investigated. The analysis of ADLs is extensively investigated in this thesis in order to understand the physical phenomena characterizing these structures. Moreover, this study also provides the guidelines to design such kind of engineered materials. The modeling of ADLs is first implemented for two-dimensional structures, and then generalized to three-dimensional geometries. A rigorous analytical formulation is presented to model ADLs of finite height by including the higher-order interaction between parallel sheets in closed form. From the equivalent impedance of the layers, a transmission line model that provides the spectral Green's function of ADLs is constructed. This allows to characterize the propagation through finite ADLs and to study their dispersion characteristics. The analysis shows that an ADL slab is equivalent to a dense material for the plane waves incident in directions close to the normal, whereas it exhibits lower effective permittivity for the waves that are incident close to the grazing angle. This peculiar anisotropic property can be exploited to design antenna superstrates which, one one hand, exhibit high surface-wave efficiency over a wide band and, on the other hand, provide high front-to-back ratio. The advantages of using ADL superstrates in the antenna designs is experimentally demonstrated by the development of two prototype demonstrators. The first design operates at X-band and is realized using commercial printed circuit board (PCB) technology. The second prototype consists of an on-chip antenna working at 0.3 THz, which is fabricated using an in-house integrated circuit process. This study highlights that the ADL superstrates can be designed and manufactured independently from the antenna and the circuit. This allows their use as add-on components, since no accurate alignment is required between the antenna and the superstrate layers. Moreover, ADL are broadband because of their non-resonant nature. The last part of this thesis deals with a novel concept of wideband wide-scanning phased array designs, based on connected arrays of slots loaded with ADL superstrates. The proposed structure has two main advantages with respect to the existing solutions. Firstly, it is completely planar and realized with a single multilayer PCB, with consequent reduction of the cost and the complexity of the array. Secondly, ADLs are used in place of real dielectric superstrates, to achieve much better efficiency in terms of surface-wave loss. For the design of the array loaded with ADL, including the adhesive layers, an analytical tool, based on spectral Green's function, is used. This tool allows to estimate the performance of the array with minimal computational resources. The design of a wideband feed structure is also proposed, which does not require balanced-to-unbalanced (balun) transitions that limit the matching bandwidth.