  In this paper we study \emph{essential hereditary undecidability}. Theories
with this property are a convenient tool to prove undecidability of other
theories. The paper develops the basic facts concerning essentially hereditary
undecidability and provides salient examples, like a construction of \ehu\
theories due to Hanf and an example of a rather natural essentially
hereditarily undecidable theory strictly below {\sf R}. We discuss the
(non-)interaction of essential hereditary undecidability with recursive boolean
isomorphism.
  We develop a reduction relation \emph{essential tolerance}, or, in the
converse direction, \emph{lax interpretability} that interacts in a good way
with essential hereditary undecidability.
  We introduce the class of $\Sigma^0_1$-friendly theories and show that
$\Sigma^0_1$-friendliness is sufficient but not necessary for essential
hereditary undecidability.
  Finally, we adapt an argument due to Pakhomov, Murwanashyaka and Visser to
show that there is no interpretability minimal essentially
hereditarilyundecidable theory.
