Abstract: Supplemental content such as electronic advertisements may be previewed in situ. That is, supplemental content may be previewed in a context of a live and/or production environment along with the primary content available in that environment. In particular, electronic advertising that is still under development, that still requires testing and/or quality control, and/or electronic advertising that is unpublished may be incorporated into electronic pages by live and/or production electronic page generation computer systems. Furthermore, electronic advertising may be previewed in situ in a controlled and/or restricted manner.