To a large extent, our knowledge of the photoinduced dynamics of molecular systems at the atomic level is shaped by nonlinear femtosecond spectroscopy. Traditionally, nonlinear femtosecond spectroscopy is an ensemble spectroscopy, performed on ensembles of identical chromophores in the gas phase or in the liquid phase. Modern femtosecond spectroscopy comprises a set of various third-order or higher-order technique including, for example, fluorescence up conversion, transient absorption, and photon echo spectroscopy. Very recently, the portfolio of femtosecond techniques has been extended towards singlemolecule detection by the development of fluorescence-detected double-pump singlemolecule spectroscopy. With this technique, a temporal resolution of about ten femtoseconds has been achieved. The technique permits the real-time monitoring of not only electronic populations, but also of electronic and vibrational coherence for individual molecules.