Argonaute proteins were first discovered genetically, and extensive research in the past few years has revealed that members of the Argonaute protein family are key players in gene-silencing pathways guided by small RNAs. Small RNAs such as short interfering RNAs (siRNAs), microRNAs (miRNAs) or Piwi-interacting RNAs (piRNAs) are anchored into specific binding pockets and guide Argonaute proteins to target mRNA molecules for silencing or destruction. Various classes of small RNAs and Argonaute proteins are found in all higher eukaryotes and have important functions in processes as diverse as embryonic development, cell differentiation and transposon silencing. Argonaute proteins are evolutionarily conserved and can be phylogenetically subdivided into the Ago subfamily and the Piwi subfamily. Ago proteins are ubiquitously expressed and bind to siRNAs or miRNAs to guide post-transcriptional gene silencing either by destabilization of the mRNA or by translational repression. The expression of Piwi proteins is mostly restricted to the germ line and Piwi proteins associate with piRNAs to facilitate silencing of mobile genetic elements. Although various aspects of Argonaute function have been identified, many Argonaute proteins are still poorly characterized. Therefore, it is very likely that as yet unknown functions of the Argonaute protein family will be elucidated in the future.