  We study the properties of reflectionless measures for an $s$-dimensional
Calder\'on-Zygmund operator $T$ acting in $\mathbb{R}^d$, where $s\in (0,d)$.
Roughly speaking, these are the measures $\mu$ for which $T(\mu)$ is constant
on the support of the measure. In this series of papers, we develop the basic
theory of reflectionless measures, and describe the relationship between the
description of reflectionless measures and certain well-known problems in
harmonic analysis and geometric measure theory.
