A new method for analyzing calculations of vertical electronic transitions in molecules is proposed. The one-electron difference density matrix between the two states is decomposed into the negative of a "detachment density" describing removal of charge from the initial state plus an "attachment" density describing its new arrangement in the excited state. This approach relates closely to the simple picture of excited states as electron promotions from occupied to unoccupied orbitals, and yet it can be applied to arbitrarily complex wave functions. The trace of the attachment and detachment densities is a measure of the number of electrons promoted in a transition. Attachment and detachment densities are calculated and analyzed for electronic transitions in formaldehyde and the nitromethyl radical.