In this manuscript we describe a fundamentally novel approach to the design of anti-aliasing filters. The approach, termed Filtering by Aliasing, incorporates the frequency-domain aliasing operation itself into the filtering task. The spectral content is spread with a periodic mixer and weighted with a simple analog filter before it aliases at the sampler. By designing the system according to the formulations presented in this manuscript, the sampled output will have been subjected to sharp, highly programmable anti-alias filtering. This manuscript describes the proposed Filtering by Aliasing idea, the effective programmable anti-aliasing filter, its design, and its range of frequency responses. The manuscript also addresses the implementation sensitivities of the proposed Filtering by Aliasing approach and provides a performance comparison against existing techniques in the context of reconfigurable anti-alias filtering.