  We study sets and groups definable in tame expansions of o-minimal
structures. Let $\mathcal {\widetilde M}= \langle \mathcal M, P\rangle$ be an
expansion of an o-minimal $\mathcal L$-structure $\cal M$ by a dense set $P$,
such that three tameness conditions hold. We prove a structure theorem for
definable sets and functions in analogy with the influential cell decomposition
theorem known for o-minimal structures. The structure theorem advances the
state-of-the-art in all known examples of $\mathcal {\widetilde M}$, as it
achieves a decomposition of definable sets into \emph{unions} of `cones',
instead of only boolean combinations of them. We also develop the right
dimension theory in the tame setting. Applications include: (i) the dimension
of a definable set coincides with a suitable pregeometric dimension, and it is
invariant under definable bijections, (ii) every definable map is given by an
$\cal L$-definable map off a subset of its domain of smaller dimension, and
(iii) around generic elements of a definable group, the group operation is
given by an $\cal L$-definable map.
