During this PhD study, metamaterials incorporating active devices such as varactors and non-Foster circuits, were researched and designed. Starting from the research on tuneable metamaterials, an electronically controlled leaky-wave (LW) antenna based on composite right/left handed (CRLH) transmission line (TL) structure was proposed which could perform a broadband beam-fixing function with the frequency range from 1 to 4 GHz. In addition, scanning from forward to backward at a fixed frequency can be achieved by manipulating the biasing voltage applied to the varactors. Most of this study has been devoted to the non-Foster active metamaterials. First, the characterization of active magnetic metamaterials with non-Foster loads was presented. Based on the equivalent circuit model, stability of an actively-loaded loop array was examined through different analysis techniques, further to give the design specifications to achieve the broadband non-dispersive negative-Re(μ) (MNG) or μ-near-zero (MNZ) magnetic properties. Moreover, the wave propagation in the actively-loaded medium was investigated. By relating the dispersion characteristics and the effective medium properties, we henceforth proposed the design of zero-loss and broadband metamaterials. This thesis also has covered the study of active high impedance surfaces (HIS) with non-Foster loads. As a two-dimensional metamaterial structure, HIS have been widely used in the microwave and antenna engineering. However it can be easily seen that the performance of a general passive HIS is always limited by the narrow bandwidth, thus making a broadband HIS desirable. In this work, an analytical solution to achieving a stable broadband HIS structure is given by incorporating appropriate negative impedance converter (NIC) circuits. Simulation results have verified the design approach. Finally, the design of NIC circuits was presented as the key part of the realization of active metamaterials. Two schemes have been adopted to realize the design of NICs, one is the operational amplifier (op-amp) based NIC, and another is based on discrete transistors. Both types of NICs were introduced and studied in this thesis.