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 developed and implemented. Typical attosecond experiments rely on a cross-correlation measurement of the combined XUV / IR fields with attosecond resolution. Attosecond pulse trains (APT) are temporally characterized by means of a RABBITT (reconstruction of attosecond beating by interference of two photon transitions) measurement. These APTs are then used in an all-optical XUV/IR pump-probe measurement in a dense helium gas target. The absorption is influenced by a strong IR field. This experiment can be regarded as the first all-optical XUV/IR pump-probe measurement with attosecond resolution.