  We define a closed model category containing the $n$-nerves defined by
Tamsamani, and admitting internal $Hom$. This allows us to construct the
$n+1$-category $nCAT$ by taking the internal $Hom$ for fibrant objects. We
prove a generalized Seifert-Van Kampen theorem for Tamsamani's Poincar\'e
$n$-groupoid of a topological space. We give a still-speculative discussion of
$n$-stacks, and similarly of comparison with other possible definitions of
$n$-category.
