  In the study of complex flag manifolds, flag domains and their cycle spaces,
a key point is the fact that the cycle space $\mathcal M_D$ of a flag domain
$D$ is a Stein manifold. That fact has a long history. The earliest approach
relied on construction of a strictly plurisubharmonic function on $\mathcal
M_D$, starting with a $q$--convex exhaustion function on $D$, where $q$ is the
dimension of a particular maximal compact subvariety of $D$ (we use the
normalization that 0--convex means Stein). Construction of that exhaustion
function on $D$ required that $D$ be measurable. In that case the exhaustion on
$D$ was transferred to $\mathcal M_D$, using a special case of a method due to
Barlet. Here we do the opposite: we use an incidence method to construct a
canonical plurisubharmonic exhaustion function on $\mathcal M_D$ and use it in
turn to construct a canonical $q$--convex exhaustion function on $D$. This
promises to have strong consequences for cohomology vanishing theorems and the
construction of admissible representations of real reductive Lie groups.
