  The monitoring of the atmosphere is very relevant for Imaging Atmospheric
Cherenkov Telescopes. Adverse weather conditions (strong wind, high humidity,
etc.) may damage the telescopes and must therefore be monitored continuously to
guarantee a safe operation, and the presence of clouds and aerosols affects the
transmission of the Cherenkov light and consequently the performance of the
telescopes. The ATmospheric CAlibration (ATCA) technical working group of the
MAGIC collaboration aims to cover all aspects related to atmosphere monitoring
and calibration. In this paper we give an overview of the ATCA goals and
activities, which include the set-up and maintenance of appropriate
instrumentation, proper analysis of its data, the realization of MC studies,
and the correction of real data taken under non-optimal atmospheric conditions.
The final goal is to reduce the systematic uncertainties in the determination
of the $\gamma$-ray flux and energy, and to increase the duty cycle of the
telescopes by establishing optimized data analysis methods specific for real
atmospheric conditions.
