In this chapter, we review the information loss paradox in the spirit of the Euclidean path integral approach. First, we argue that there is a long debate about the information loss paradox, and the non-perturbative quantum gravitational wave function must include the clue to the paradox. The Euclidean path integral approach provides the best way to describe the wave function. From this wave function, we can notice that there are not only semi-classical but also non-perturbative contributions, which are highly suppressed but preserved information. Information retrieval will be sufficiently explained if such non-perturbative contributions must be dominated by the late time. We will show that there is sufficient evidence that this scenario can be realized in generic circumstances. Finally, we compare this scenario with alternative approaches. Also, we comment on some unresolved issues that need to be clarified.