Relative perfect complexes were defined already in SGA 6 seminar alongside its absolute counterpart. The original definition and characterization is somehow awkward. Absolute perfect complexes had played a very important role in recent studies in cohomology of algebraic varieties. In this talk we will discuss a new characterization of relative perfect complexes among pseudo-coherent complexes and give some applications. As an example, we will explain a non-noetherian generalization of the semicontinuity theorem.Zoom notes: Meeting ID 995 3589 3289