The origin of magnetic fields is one of the great, fundamental mysteries of astrophysics. Global magnetic fields in planets, in the Sun and other stars, in spiral galaxies and galaxy clusters are believed to be generated and maintained by a hydromagnetic dynamo, a process that converts turbulent kinetic energy into magnetic energy. The dynamo processes operate on drastically different scales, but are associated with common physical mechanisms, involving the complex interactions of rotation, turbulence and MHD instabilities. Numerical simulations of planetary dynamos are increasingly successful in reproducing details of the geomagnetic field and making predictions for other planets. Also, substantial progress is made in both observations and theory of the dynamo processes in the Sun and other objects.

The 20 years following the first IAU Symposium dedicated to dynamo processes (IAU Symposium 157 "The Cosmic Dynamo", Potsdam, 1992) have witnessed the founding of a new field of astronomy; ideas developed by solar astronomers have been successfully applied to other stars and astrophysical objects, in which entirely new magnetic activity phenomena have been discovered. Solar physicists have used helioseismology to greatly constrain the mechanisms that generate magnetic fields inside the Sun. High-resolution observations of surface magnetism and coronal dynamics have provided new knowledge about magnetic relaxation processes in astrophysical conditions. The connection between the interior dynamo processes and the outer magnetized coronae is a new, cross-disciplinary aspect to the solar-stellar magnetism investigations. The theoretical framework for solar activity has proven to be a surprisingly robust framework for discussing the activity of other stars, accretion disks, galaxies, and interstellar medium. However, the number of new questions, that new observations have given birth to, is much larger than the number of questions previously answered.

The Sun and stars exhibit a variety of enigmatic phenomena that continue to be inscrutable and defy our understanding. We do not understand how the magnetic fields are generated within the Sun and stars, how the Sun’s intense magnetism is concentrated into sunspots as large as the Earth, how the dynamo-generated magnetic fields trigger solar and stellar activity, how the ubiquitous small-scale magnetic fields are organized on larger scales and relate to the dynamo process, how we can predict the solar and stellar activity cycle, and, in particular, why the dynamo produces the solar and stellar activity cycles with long activity minima. The unusual long minimum and slow onset of Solar Cycle 24 defied theoretical predictions and posed new challenges for solar-cycle dynamo theories.

Recently, several important solar instruments have attracted the interest of solar and stellar astronomers all over the world. These include the existing space observatories: TRACE, RHESSI, STEREO, Hinode, PICARD, and Solar Dynamics Observatory (SDO); balloon observatory SUNRISE; large ground-based telescopes: Swedish Solar Telescope (SST), New Solar Telescope (NST), Gregor, and the constructing Advanced Technology Solar Telescope (ATST) and Chinese Spectral Radioheliograph (CSRH). We now have the means to explore the secrets of solar activity and their periodicity by using a wide range of multi-wavelength observations from the ground and space. Solar magnetic activity includes flares, coronal mass ejections (CME), eruptive filaments, outbursts of varying spatial scales and long-term variation. Coronal phenomena are driven by the dynamo-generated fields that show large-scale organization. Coronal magnetic activity is a crucial mechanism for the magnetic relaxation and transport of magnetic helicity continuously replenished by the dynamo. At the same time, many types of stars produce a variety of magnetic activity. The vastly different physical conditions on stars can help to shed light on processes operating on the Sun. Substantial progress has been made in Doppler imaging and spectro-polarimetric techniques. Asteroseismology observations from CoRot and Kepler space missions provide new insights into the stellar structure, dynamics and variability. On the other hand, the Sun allows us to study the dynamo and magnetic activities in more detail than any study of stellar activities. Therefore, solar research becomes highly relevant to further an understanding of the activity of other stars. Thus, the solar-stellar connection is absolutely crucial for the solar and stellar communities alike. Achievements in understanding magnetic dynamos will greatly enrich our knowledge of the Earth’s and planetary magnetic environments as well.

Very similar dynamo processes play a key role in the magnetism of other astrophysical objects. Planets with liquid cores produce magnetic fields by a dynamo mechanism. This mechanism is currently best understood among all the astrophysical dynamos thanks to advanced numerical simulations. The simulations have been able to explain the great variety of planetary magnetic fields and have provided an understanding of how the observations connect to the dynamo models. Thus, important lessons for other astrophysical dynamos can be learned from these studies.

On galactic scales, the role of the dynamo is much less understood. Radio observations of polarized synchrotron emission provide important three-dimensional maps of the strength, structure and turbulent properties of the magnetic field in galaxies, clusters and interstellar medium. Planned new facilities, such as the Square Kilometer Array (SKA), Expanded Very Large Array (EVLA), Five-hundred-meter Aperture Spherical radio Telescope (FAST), LOw Frequency ARray (LOFAR), and also gamma-ray Cherenkov Telescope Array (CTA) will substantially advance our knowledge of galactic magnetism. Such observations are very challenging but will provide unobstructed views of the dynamo operating on a large cosmic scale. From the theoretical point of view, understanding the dynamo mechanism in such conditions is also very challenging. Most of the magnetic field structures in the galaxies require a superposition of several dynamo modes to explain their origin, amplification, and large-scale organization of intermittent magnetic fields. In some galaxies, the halo rotates slower than the disk providing a velocity gradient that could lead to excitation of a global dynamo in disk galaxies in a similar way to the differential rotation in the solar dynamo. Thus, important lessons can be learned from the cross-disciplinary discussions of the dynamo mechanism in such extreme, galactic conditions compared to the solar dynamo. This symposium will be an important step towards solving the great puzzle of the origin, regeneration and maintenance of astrophysical magnetic fields.