What are all-pass filters and their applications?

All-pass filters have unity magnitude response at all frequencies but non-constant phase: |H(e^jw)| = 1 for all w. Transfer function has poles and zeros in conjugate reciprocal pairs (z0 and 1/z0*). Applications: Phase equalization (compensate nonlinear phase of minimum-phase systems), Group delay equalization (flatten varying group delay), Phase shifters (specific phase shifts at target frequencies), and Building blocks for filter design (lattice/ladder structures). Cascade of all-pass sections can create arbitrary phase response while maintaining magnitude.