Cosmic masers have been studied since the discoveries of the first interstellar molecules of OH and H2O in 1960’s. Masers are known to be associated with various objects, predominantly young stellar objects (YSOs), evolved stars, in particular asymptotic giant branch (AGB) and post-AGB stars, and active galactic nuclei (AGNs). Most masers emit high brightness, narrow spectral features, often over a wide velocity range from compact regions that, in the Galaxy, have sizes of order a few au and line widths of 0.1 km/s. Thus, masers can be imaged with high spatial/spectral resolutions with radio interferometers and are the only spectral emission lines detectable with very long baseline interferometry (VLBI).

   In terms of research topics, the maser scientific community is diverse and multidisciplinary but has long been tied together through the common background physics and observational techniques. An important method for the community to connect and build new international collaborations is through regular meetings. Over the past decades, there have been IAU meetings on cosmic masers spaced by 5-9 years, and the last of these was held in 2017. During that meeting, more than 30 attendees from 10 countries recognized the importance of sudden, strong brightening of masers (flare or burst), and an international collaboration network, the Maser Monitoring Organization (M2O) was formed, which has grown to 100 members by now. This new systematic pursuit of time-domain maser science has become a powerful means to investigate episodic mass accretion and ejection in YSOs, since masers are sensitive to rapid variations of physical conditions.

   Following these past activities, we will organize the next maser symposium in Kagoshima, Japan. Since the first major maser conference in the USA (1992), past IAU meetings on masers have been hosted in Brazil (2001), Australia (2007), South Africa (2012), and Italy (2017). Thus, organizing this IAUS on masers in Asia will close a gap and such symposia will have taken place in all of the world’s major regions.

   It has been almost six years since the last IAUS on cosmic masers. During this period, the Atacama Large Millimeter/submillimeter Array has achieved 10s of milli-arcsecond (mas) resolution, which is comparable to present VLBI maser maps. In addition, ALMA opened new windows for unseen high frequency masers. Several large programs with ALMA and other facilities appeared during this period and in the near future will enable us to link the VLBI maser maps with thermal spectral line and continuum images for the first time, and will fill the large gaps from compact (mas) to extended (degree) structures. Furthermore, the Square Kilometre Array (SKA) has just started construction in 2022, and pilot surveys are ongoing with its precursors such as MeerKAT and ASKAP in South Africa and Australia, respectively. The next generation Very Large Array (ngVLA) will become complementary to SKA and ALMA and has just been strongly endorsed by the US decadal survey Astro2020. Masers science cases are proposed in these projects’ white papers published since the last IAUS. As for infrared and optical wavelengths, a huge amount of astrometric data  from Gaia can be compared with VLBI maser astrometry at a comparable accuracy of 0.01 mas. For the future projects, the James Webb Space Telescope (JWST) also has just been launched and other ground-based instruments such as the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT) are also under development. Discussion on their possible science cases and synergies will be timely and hence, covered in the proposed meeting.

   While the main focus of this IAUS is on cosmic masers, we intend to invite speakers from outside of the maser communities to broaden the impact of the IAUS through synergies with other research fields. Theoretical reviews of forefront questions of cosmic maser science are planned. The forthcoming IAUS will offer the opportunity to discuss advances in observation and theory, and to plan challenging programs in maser research that will tackle fundamental scientific questions of the next decade in global collaboration.