  Quantum mechanics is one of our most successful physical theories; its
predictions agree with experimental observations to an extremely high accuracy.
However, the bare formalism of quantum theory does not provide straightforward
answers to seemingly simple questions: for example, how should one model
systems that include agents who are themselves using quantum theory? These
foundational questions may be investigated with a theorist's tool -- the
thought experiment. Its purpose is to turn debates about the interpretation of
quantum theory into actual physics questions. In this article, we give a
state-of-the-art overview on quantum thought experiments involving observers,
from the basic Wigner's friend to the recent Frauchiger-Renner setup,
clarifying their interpretational significance and responding to objections and
criticism on the way.
