The directed diffusion module in ns has been ported from SCADDS group's implementation of directed diffusion at USC/ISI. There is an older version of diffusion in ns that was implemented several years back and has become relatively old and outdated. This older version can be found under directory diffusion. And the newer version of diffusion resides under ~ns//diffusion3. This chapter talks about the newer diffusion model in ns. The module and methods described here can be found under ~ns/tcl/lib/ns-diffusion.tcl, ns-lib.tcl and all relevant C++ code can be found under ~ns/diffusion3. Visit the SCADDS group webpage at http://www.isi.edu/scadds for details about their implementation.