The connection graph proof procedure (or clause graph resolution as it is more commonly called today) is a theorem proving technique due to Robert Kowalski. It is a negative test calculus (a refutation procedure) based on resolution. Due to an intricate deletion mechanism that generalises the well-known purity principle, it substantially refines the usual notions of resolution-based systems and leads to a largely reduced search space. The dynamic nature of the clause graph upon which this refutation procedure is based, poses novel meta-logical problems previously unencountered in logical deduction systems. Ever since its invention in 1975 the soundness, confluence and (strong) completeness of the procedure have been in doubt in spite of many partial results. This paper provides an introduction to the problem as well as an overview of the main results that have been obtained in the last twenty-five years.