  We review the main research directions ongoing in the development of
high-harmonic generation-based extreme ultraviolet sources for the
synthesization and application of trains and isolated attosecond pulses to
time-resolved spectroscopy. A few experimental and theoretical works will be
discussed in connection to well-established attosecond techniques. In this
context, we present the unique possibilities offered for time-resolved
investigations on the attosecond timescale by the new Extreme Light
Infrastructure Attosecond Light Pulse Source, which is currently under
construction.
