The theory of the solar/stellar activity cycles is presented, based on the mean-field concept in magnetohydrodynamics. A new approach to the formulation of the electromotive force as well as the theory of differential rotation and meridional circulation is described for use in dynamo theory. Solar dynamo theory is reviewed in the special context of the cycle-time problem. Differential rotation is also turbulence-induced but without the complications due to the magnetic fields. The main observational features of the solar differential rotation are surface equatorial acceleration of about 30%, strong polar sub-rotation and weak equatorial super-rotation and reduced equator-pole difference in Ω at the lower convection-zone boundary. Toroidal magnetic fields are generated by differential rotation in the overshoot layer whereas a dynamo acts in the upper part of the convection zone. The plane has infinite extent in the radial (x) and the azimuthal (y) direction; the boundaries are in the z-direction.