We have measured the collective excitation spectrum of interacting electrons in one-dimension. The experiment consists of controlling the energy and momentum of electrons tunneling between two clean and closely situated, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. At high elelctron densities the measured excitation spectrum clearly deviates from the non-interacting spectrum, attesting to the importance of Coulomb interactions. Notable is the observation of two excitation branches corresponding to spin - charge separation. In short wires, 6 microns and 2 microns long, finite size effects, resulting from breaking of translational invariance, are observed. Here spin and charge separation is manifested through Moire patterns generated from the spin and charge excitation velocities. At low electron densities the system abruptly looses translation invariance and becomes localized. We find that the localization length corresponds to the inter-electron spacing determined by the 1D electron density.