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In the early 1980s, Carl Sagan and Frank Tipler published a series of articles in the pages of the Quarterly Journal of the Royal Astronomical Society on ETI that became a cause celebre at the time. Whilst reading for an MSc in Astronomy at the University of Sussex in the early 1990s, I expressed my interest in SETI as a staunch Saganite (influenced not only by the phenomenal Cosmos TV series, recently re-vamped by Neil de Grasse Tyson, but also by Sagan's preceding Royal Institution Christmas lecture series) to Professor John Barrow who introduced me to the relevant chapter in the book The Anthropic Cosmological Principle that he co-authored with Frank Tipler (I still recommend the entire book for its visionary scope). I immediately went to source material and was struck by the von Neumann or self-replicating probe concept under discussion (I shall use the latter term to avoid conflation with von Neumann machine which has a well-established meaning in computer engineering). This was my road to Damascus in converting me from ETI believer to skeptic (but by the power of scientific argument rather than any epiphany). Thence, I decided to pursue a PhD in space engineering specialising in space robotics (though my research project was rather more mundane than self-replicating probes). Yet, despite its foundational importance, the Sagan-Tipler debate is almost forgotten today despite the fact that it exposes the root-and-branch of the SETI venture. However, every now and then, there is still the occasional paper on self-replicating probes but they appear to be sidestream to the SETI programme. This special issue seeks to re-focus the self-replicating probe back into the SETI mainstream where it belongs.