a delayed XUV attosecond pulse, with a following motion in the continuum states driven by a strong IR field. The resulting 2D photo-electron (momentum, or equivalently energy, vs delay) distribution is called streaking trace. Attosecond techniques Here are listed and discussed some of the most common techniques and approaches pursued in attosecond research centers. Attosecond metrology with photo-electron spectroscopy (FROG-CRAB) A daily challenge in attosecond science is to characterize the temporal proprieties of the attosecond pulses used in any pump-probe experiments with atoms, molecules or solids. The most used technique is based on the frequency-resolved optical gating for a complete reconstruction of attosecond bursts (FROG-CRAB). The main advantage of this technique is that it allows to exploit the corroborated FROG technique, developed in 1991 for picosecond-femtosecond pulse characterization, to the attosecond field. CRAB is an extension of FROG and it is based on the same idea for the field reconstruction. In other words, FROG-CRAB is based on the conversion of an attosecond pulse into an electron wave-packet that is freed in the continuum by atomic photo-ionization, as already described with Eq.. The role of the low-frequency driving laser pulse( e.g. infra-red pulse) is to behave as gate for the temporal measurement. Then, by exploring different delays between the low-frequency and the attosecond pulse a streaking trace (or streaking spectrogram) can be obtained. This 2D-spectrogram is later analyzed by a reconstruction algorithm with the goal of retrieving both the attosecond pulse and the IR pulse, with no need of a prior knowledge on any of them. However, as Eq. pinpoints, the intrinsic limits of this technique is the knowledge on atomic dipole proprieties, in particular on the atomic dipole quantum phase. The reconstruction of both the low-frequency field and the attosecond pulse from a streaking trace is typically achieved through iterative algorithms, such as: Principal component generalized projections algorithm (PCGPA). Volkov transform generalized projection algorithm (VTGPA). extended ptychographic iterative engine (ePIE). See also High harmonic generation Femtochemistry Femtotechnology Ultrashort pulse Chirped pulse amplification Free-electron laser Attosecond chronoscopy References Further reading Attosecond groups in the world University of Nebraska-Lincoln Max Planck Institute of Quantum Optics, Munich Lund University ELI-ALPS Szeged Stanford University Politecnico Milano CEA Paris Max Born Institute, Berlin University of Freiburg JAS Lab Ottawa Imperial College London CFEL Hamburg ICFO Barcelona OSU Columbus ETH Zurich FORTH Heraklion Griffith University, Queensland Weizmann Institute of science, Tel Aviv RIKEN Tokyo Heidelberg University