We developed an importer from both HOL 4 and HOL-light into Isabelle/HOL. The importer works by replaying proofs within Isabelle/HOL that have been recorded in HOL 4 or HOL-light and is therefore completely safe. Concepts in the source HOL system, that is types and constants, can be mapped to concepts in Isabelle/HOL; this facilitates a true integration of imported theorems and theorems that are already available in Isabelle/HOL. The importer is part of the standard Isabelle distribution.