"An Architecture for Plug-in Tutor Agents" (Ritter and Koedinger 1996) proposed a software architecture designed around the idea that tutors could be built as plug-ins for existing software applications. Looking back on the paper now, we can see that certain assumptions about the future of software architecture did not come to be, making the particular approach described in the paper infeasible. However, the pedagogical approach assumed by the architecture remains relevant today, and the basics of the architecture are applicable in purpose-built instructional systems. [For "An Architecture for Plug-In Tutor Agents," see EJ539731.]