Contents 
We consider variants of the KellerSegel system of chemotaxis which contain logistictype source terms and thereby
account for proliferation and death of cells. We briefly review results and open problems with regard to the fundamental
question whether solutions exist globally in time or blow up.
The primary focus will then be on the prototypical parabolicelliptic system
\[
\begin{array}{l}
u_t=\varepsilon u_{xx}  (uv_x)_x + ru  \mu u^2,
0= v_{xx}v+u,
\end{array} \right.
\]
in bounded real intervals. The corresponding Neumann initialboundary value problem, though known to possess global bounded
solutions for any reasonably smooth initial data, is shown to have the property that the socalled
{\em carrying capacity} $\frac{r}{\mu}$ can be exceeded dynamically to an arbitrary extent during evolution in an
appropriate sense, provided that $\mu0$ is sufficiently small.
This is in stark contrast to the case of the corresponding Fishertype equation obtained upon dropping the term $(uv_x)_x$,
and hence reflects a drastic peculiarity of destabilizing action due to chemotactic crossdiffusion, observable
even in the simple spatially onedimensional setting.
Numerical simulations underline the challenge in the analytical derivation of this result by indicating that
the phenomenon in question occurs at intermediate time scales only, and disappears in the large time asymptotics. 
