  Blazars are known to display strong and erratic variability at almost all the
wavelengths of electromagnetic spectrum. Presently, variability studies at
high-energies (hard X-rays, gamma-rays) are hampered by low sensitivity of the
instruments. Nevertheless, the latest generation of satellites (INTEGRAL,
Swift) have given suggestions not yet fully confirmed of variability on
intraday timescales. Some specific cases recently observed are presented and
physical implications are discussed (e.g. NRAO 530). The contribution that
SIMBOL-X could give in this research topic is also outlined.
