The Autotune Filter (AtF) is a novel passive electromagnetic interference device. It provides suppression of multiple simultaneous interfering signals while small signals within its instantaneous operating band pass through unaffected. This capability effectively enhances the dynamic range of receivers, enabling them to process a much wider power range of input signals than previously. This technical brief describes the basic functionality of autotune filter and introduces several variations and performance metrics of the technology. These devices are particularly useful in communications and sensing to help ruggedize receivers against the increasingly congested and rapidly evolving electromagnetic spectrum.