In reguards to Magic the Gathering, at what point does a player have priority in their opponents turn? MTG?

Pretty much, at what point can I stop my opponent and say, "in respone" and cast a spell. I know I can when the opponent plays a spell and its resting on the stack, but do I also have one when they enter a step or phase, declare attackers or blockers, or even at the very end of their turn?