This work describes experiments aiming at the observation of ultrafast dynamics of photo-excited electrons in condensed matter that evolves on a sub-femtosecond time scale. The experimental scheme providing this unprecedented time resolution draws on the simultaneous illumination of a solid surface with the electric fields of two different light pulses: photoelectrons generated by an extreme-ultraviolet (XUV) pulse, with only a few hundred attoseconds in duration, are modulated in their final kinetic energy by a precisely synchronized, phase-controlled light field of a near-infrared femtosecond laser pulse. A detailed analysis of these modulations for electrons ejected from different electronic states of the solid allows access to relative time delays occurring during the photoemission process. The capability of this method for clocking electron emission from metal surfaces with a precision of only a few attoseconds is demonstrated. In the quest of exploring the origin of these temporal phenomena in solid-state photoemission, experiments on different single crystals and well-defined metal-adsorbate interfaces have been performed. These measurements emphasize the extreme sensitiv