Since the development of the first Synthetic Aperture Radar (SAR), radar hasbecome an important sensor for imaging applications. Manifold fields of ap-plication ranging from climate change research, over change-detection, and 4-Dmapping up to earthquake and flood monitoring are covered by SAR. Especiallyhigh-resolution, day- and night-, as well as weather-independent imaging capa-bilities led to the success of recent spaceborne SAR missions. However, due to ahigh demand for global SAR data sets, state-of-the-art sensors reach their limitsin terms of resolution, swath-width, repeat cycle, and flexibility.To solve SAR-inherent limitations, in this dissertation, a new generation of SARsensors with multiple transmit and multiple receive channels (MIMO) and Digi-tal Beam-Forming (DBF) capabilities is proposed. Apart from an increased flex-ibility, this enables a large variety of new operation- and acquisition-modes forhigh-resolution wide-swath SAR imaging.To separate the individual channels in the post-processing, established pro-cessing techniques can not be applied, since SAR is highly sensitive to interfer-ences. Hence, the Space-Time-Frequency Adaptive Processing(STFAP) algorithmis derived, which allows channel separation without any interferences. Basically,STFAP forms frequency- and time-variant antenna beams, which follow the echosignals of each transmitted waveform individually. However,this method de-mands waveforms of certain structures, which are described and suitable wave-form types are compared in detail.STFAP and other DBF-SAR techniques need very sharp antenna beams in eleva-tion. In presence of unknown topography, when the assumed geometrical modelof the Earth surface is complex, antenna pointing might be mismatched with thesurface geometry. To handle this issue, it is suggested to apply an additional al-gorithm prior to DBF, which determines the angle of the incident echo signal onthe antenna array. The algorithm, which is proposed in this dissertation, is basedon the matrix pencil method and allows a highly accurate and signal-adaptiveDBF in real-time.For an experimental verification of new imaging modes and STFAP, a ground-based MIMO-SAR demonstrator in X-band has been developed. The dissertationconcludes with its description and promising experimental results, which proofthat the suggested concepts have the potential to enhance the capabilities of state-of-the-art SAR sensors significantly.