Argonaute proteins (Agos) use small 15–30 nucleotide-long guides to bind and/or cleave complementary target nucleic acids. Eukaryotic Agos mediate RNA-guided RNA silencing, while 'long' prokaryotic Agos (pAgos) use RNA or DNA guides to interfere with invading plasmid and viral DNA. Here, we review the function and mechanisms of truncated and highly divergent 'short' pAgos, which, until recently, remained functionally uncharacterized. Short pAgos have retained the Middle (MID) and P-element-Induced Wimpy Testis (PIWI) domains important for guide-mediated target binding, but lack the ability to cleave their targets. Instead, emerging insights reveal that various short pAgos interact with distinct accessory 'effector' enzymes. Upon guide-mediated detection of invading DNA by short pAgos, their associated effector enzymes kill the host cell and, consequentially, prevent spread of the invader. In all domains of life, Argonaute proteins (Agos) use short nucleic acid guides to bind complementary target nucleic acids in a sequence-specific manner. Whereas eukaryotic Agos mediate RNA-guided RNA targeting, prokaryotic Agos (pAgos) show distinct guide/target preferences (DNA/RNA), have varied domain compositions, and associate with a wide range of auxiliary proteins. Not only have full-length pAgos (long pAgos) been shown to target and degrade invading DNA, but also the role and mechanisms of the more abundant truncated 'short' pAgos have recently been elucidated. Short pAgos have lost the ability to cleave targets; instead they genetically associate with a wide variety of putative effector proteins, including nucleases, NAD(P)ases, and depolarization-inducing membrane enzymes. Rather than directly degrading their targets, characterized short pAgos form complexes with their associated effector enzymes to trigger cell death upon guide-mediated recognition of invading DNA and, thus, function as abortive infection systems. [ABSTRACT FROM AUTHOR]