  Multiband systems, which possess a wide parameter space, allow to explore a
variety of competing ground states. Bright examples are the Fe-based pnictides
and chalcogenides, which demonstrate metallic, superconducting, and various
magnetic phases. Here I discuss only one of the many interesting topics,
namely, spin fluctuations in metallic multiband systems. I show how to
calculate the effect of itinerant spin excitations on the electronic properties
and formulate a theory of spin fluctuation-induced superconductivity. The
superconducting state is unconventional and thus the system demonstrates
unusual spin response with the spin resonance feature. I discuss its origin,
consequences, and relation to experimental observations. Role of the spin-orbit
coupling is specifically emphasized.

Superconductivity