DTD vs Schema....

Hi All,
  can some body tell me how to Determine when to use DTD versus XML Schema
w.r.t legacy systems, scoping, run-time modification, constraints

Thanx in Advance 


RaviShanker Tippa,
Programmer Analyst,
Syntel India Ltd,
21 MountRoad,Saidapet,
Chennai-15
Ph:2300368 ext-35