Random walk on discrete lattices is a fundamental model in physics that forms
the basis for our understanding of transport and diffusion process. In this
work, we study unreachability in networks, {\it i.e}, the number of nodes not
visited by any walkers until some finite time. We show that for the case of
multiple random walkers on scale-free networks, the fraction of sites not
visited is well approximated by a stretched exponential function. We also
discuss some preliminary results for distinct sites visited on time-varying
networks.