The principles of operation of a double barrier resonant tunneling diode (DBRTD) giving rise to negative differential conductivity effect are first reviewed. Next, the physics of resonant tunneling based on (i) the time-independent conventional approach and (ii) the time-dependent quantum measurement approach, as applied to a DBRTD, is discussed. Expressions for the resonant tunneling current densities through the barriers are then derived on the ideas of quantum measurement. Through the well the current, however, flows by the conventional mechanism. The three current density magnitudes are found to be identical under resonant conditions. Finally, an expression for the resonant tunneling current density due to a group of incident electrons is derived.