1 Introduction

The multitude of activity phenomena on the Sun are related to magnetic fields which are generated by cyclonic turbulence in the outer convection zone and penetrate the solar atmosphere forming sunspots, plages, network, etc. They further expand into the outer atmosphere and exhibit themselves as highly dynamic coronal loops. Thus, a detailed study of solar activity phenomena reveals the structure of underlying magnetic fields and provides valuable constraints for solar dynamo theory. These same activity phenomena are observed on cool stars with outer convection zones.

Studying magnetic activity on stars other than the Sun provides an opportunity for detailed tests of solar dynamo models. Using only solar observations limits the range of the global stellar parameters for such tests, while an extensive sample of stars of various activity levels provides key constraints for stellar and solar dynamo theory.

Stellar activity similar to that of the Sun was first discovered on red dwarfs, a fraction of which exhibit remarkable magnetic activity registered through observations of extremely strong optical flares (UV Cet-type stars). Periodic brightness variations were observed in binary systems of red dwarfs (BY Dra-type) as distortions of the light curves outside eclipses. Kron (1947 ) was seemingly the first who considered the hypothesis that spottedness of the stellar surface was causing these distortions. Later, light-curve variations due to starspots and other magnetic phenomena were discovered on various types of stars.

It was suggested by Skumanich (1972 ) that rotation plays a crucial role in the generation of stellar activity. This became evident later from the strong correlation of magnetic activity indicators with stellar rotational velocities and periods. Such relations have been reported between rotation and coronal emission (Pallavicini et al., 1981 ; Walter and Bowyer, 1981 ), chromospheric $Ca II$ and $Ha$ emission (Vaughan et al., 1981 ; Middelkoop, 1981 ; Mekkaden, 1985 ), ultraviolet line fluxes (Vilhu, 1984 ; Simon and Fekel, 1987 ), and radio emission (Drake et al., 1989 ). It was found that cool stars with more rapid rotation show a higher level of magnetic activity. Such active stars are the best choice for testing and developing stellar dynamo theory. Among single stars these are pre-main-sequence stars (T Tau-type) and early-age main-sequence stars of solar type. Evolved binary components which are tidally locked at fast rotation by a close companion are also strongly magnetically active (RS CVn-type, BY Dra-type, W UMa-type, and Algol-like systems). Rapidly rotating single giants of FK Com-type, which are probably formed from coalesced binaries complete the selection of magnetically active stars. An overview of magnetic phenomena on such stars is given in Section 2.

Significant progress in observational tools and diagnostic techniques for studying starspots over the last two decades has advanced our understanding of the nature of stellar activity. Long-term traditional photometric observations reveal active region evolution and stellar activity cycles. High-resolution spectroscopy allows for studies of the structure of active regions and stellar differential rotation with the help of the Doppler imaging technique (Vogt et al., 1987 ; Rice et al., 1989 ; Piskunov et al., 1990 ; Collier Cameron, 1992 ; Jankov and Foing, 1992 ; Berdyugina, 1998 ; Rice and Strassmeier, 2000 ). Observations of molecular lines provide a unique opportunity for insight into spatially unresolved starspots (Berdyugina, 2002 ) and unambiguous measurements of starspot temperature (O’Neal et al., 1996 ). Spectropolarimetry reveals the distribution of magnetic fields on the stellar surface thanks to the Zeeman-Doppler imaging technique (Donati et al., 1997 ). Novel applications of (spectro-)interferometry, microlensing, asteroseismology, etc., broaden the arsenal of tools and techniques for studying starspots. These are reviewed in Sections 3 and 4.

Since the discovery of rotationally modulated brightness variations due to starspots, a large amount of data has been collected for different types of stars. Brightness and colour variations allow for determining temperature of starspots and their relative area. Doppler images reveal spot distribution which is different from that observed on the Sun. Polarimetric measurements of starspots help to investigate the nature of the underlying magnetic fields. Starspot properties, including their temperature, sizes, magnetic field strengths, lifetimes, and distribution over latitudes and longitudes are reviewed in Section 5.

Time-series observations over decades reveal stellar cycles similar to the 11-year sunspot cycle. On the other hand, persistent active longitudes and a new type of stellar cycle related to them (flip-flop cycle) have been first discovered on cool stars and later on the Sun (Berdyugina and Tuominen, 1998 ; Berdyugina and Usoskin, 2003 ). In binaries, orbital period variations suggest long-term changes of the magnetic field distribution in stellar interiors (Hall, 1991b ; Rodonò et al., 1995 ; Lanza et al., 1998a ). Such observations provide valuable constraints for stellar and solar dynamo models. An overview of stellar activity cycles and their implications for dynamo theory is given in Sections 6 and 7.

Previous short reviews on starspots and their role in understanding the stellar dynamo were given by Hall (1991a ), Lanza and Rodonò (1999a ), Berdyugina (2004 ) and Strassmeier (2005 ). Also, starspots were thoroughly discussed at the international workshop “Surface Inhomogeneities on Late-Type Stars” (1990, Armagh), IAU Symposium 175 “Stellar Surface Structures” (1995, Vienna) and the First Potsdam Thinkshop “Sunspots and Starspots” (2002, Potsdam). The present review covers main tendencies in starspot research for the last two decades.