Microwave/millimeter-wave imaging systems have become ubiquitous and have found applications in areas like astronomy, bio-medical diagnostics, remote sensing, and security surveillance. These areas have so far relied on conventional imaging devices (employing Nyquist principles) which at best can provide diffraction-limited images. With the advent of metamaterials, unique and extraordinary electromagnetic responses can be achieved which can potentially revolutionize imaging devices. Such extraordinary responses include: negative refraction, strong anisotropy, gradient-index response, perfect absorption, magneto-electric effects (chirality), and many more. When adopted into imaging devices, these response characteristics could potentially: beat diffraction-limits, improve imaging performance, or lead to unprecedented control over light propagation. Along with metamaterials, mathematical tools like transformation optics or torsion optics (which leverages Riemannian geometry under the geometrical optics limit) facilitate the design and development of systems with exclusive effects such as invisibility cloaking, perfect and/or aberration-free lensing, near-field magnification, and total control of the polarization field. When metamaterial devices are combined with computational imaging techniques, the resulting systems can exploit apriori information to reach previously unattainable trade-off positions in the space of: image quality, size, weight, power, and cost. In this dissertation we present and discuss metamaterial-based imaging devices, and associated principles and techniques that achieve such enhanced imaging performance.