High-order harmonics are a unique tool to investigate dynamics in atomic and molecular systems as they allow the generation of pulses with attosecond duration. In this thesis, high-order harmonics are investigated experimentally in temporal and spectral domain. In spectral domain, an interference structure between the two shortest electron quantum paths of the generation process is observed. These interferences are investigated with laser intensities below, around and above the intensity of barrier suppression. To enable attosecond time-resolved measurements, a versatile beamline is...