We present a new list of thorium and argon emission lines in the visible obtained by analyzing high-resolution (R=110000) spectra of a ThAr hollow cathode lamp. The aim of this new line list is to allow significant improvements in the quality of wavelength calibration for medium- to high-resolution astronomical spectrographs. We use a series of ThAr lamp exposures obtained with the HARPS instrument (High Accuracy Radial-velocity Planet Searcher) to detect previously unknown lines, perform a systematic search for blended lines and correct individual wavelengths by determining the systematic offset of each line relative to the average wavelength solution.