The landscape of active regions in the solar atmosphere is marked by extended, bright patches named plage regions as well as loop-like structures which trace magnetic field lines that confine plasma several megameters above the solar surface. Within these environments, it is also common to observe different types of small-scale, short-lived optical and ultraviolet brightenings, which have been linked to magnetic reconnection processes in the low atmosphere. Therefore, solar active regions can be regarded as plasma laboratories for the study of the interplay between gas dynamics and magnetic fields. However, progress has been somewhat hampered partly due to the difficulty in interpreting the most widely used spectral diagnostics.

In this seminar I will discuss how a new observing window opened by the Atacama Large Millimeter/submillimeter Array (ALMA) can be used to address many unsettled questions in solar physics and, particularly, I will show some of the first results of solar active region studies with ALMA. The diagnostic capabilities of the millimeter continuum for both hot and cold regions and for persistent heating in plage and transient phenomena in flux emergence regions will be highlighted. I will also explore the synergy between ALMA and other facilities observing at different wavelengths.