  The electron density $n(\rb,t)$, which is the central tool of time-dependent
density functional theory, is presently considered to be derivable from a
one-body time-dependent potential $V(\rb,t)$, via one-electron wave functions
satisfying a time- dependent Schr\"{o}dinger equation. This is here related via
a generalized equation of motion to a Dirac density matrix now involving $t$.
Linear response theory is then surveyed, with a special emphasis on the
question of causality with respect to the density dependence of the potential.
Extraction of $V(\rb,t)$ for solvable models is also proposed.
