The functionality of any ferroic material depends on its domains. Consequently, their shape and manipulation in external fields are of major interest. In compounds uniting magnetic and electric order in the same phase, the magnetoelectric coupling on the level of the domains is, however, largely unexplored. For such so-called multiferroics it is therefore not known how exactly the electric or magnetic fields affect the multiferroic domains and their walls. In my talk I will discuss this issue. In particular, I will discuss magnetic-field control of a ferroelectric domains and vice versa. I will show that domain walls in multiferroics can be considered as new, mobile type of functional ferroic interface. Finally, I will present an example of all-optical control of a multiferroic state. The core technologies for us to access to multiferroic domains and their walls is nonlinear optics, here in the form of doubling of a laser frequency in a material..