  We provide an overview of the effects of interactions in Bose-condensed
gases. We focus on phenomena that have been explored in ultracold atom
experiments, covering both tuneable contact interactions and dipolar
interactions. Our discussion includes: modifications to the ground state and
excitation spectrum, critical behaviour near the Bose--Einstein condensation
temperature, the unitary regime where the interactions are as strong as allowed
by quantum mechanics, quantum droplets in mixtures, and supersolids in dipolar
gases.
