Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) genes provide adaptive immunity against invasion of foreign nucleic acids in archaea and bacteria. The system functions in three distinct stages: adaptation, biogenesis, and interference. CRISPR-Cas systems are currently classified into at least five different types, each with a signature protein among which Type III systems exhibit a dual DNA/RNA interference activity. Structures of a few Type III surveillance complexes have been determined: they are composed of several different subunits and exhibit striking architectural similarities to Type I surveillance complexes. Here, we review the genetic, biochemical, and structural studies concerning CRISPR-Cas Type III systems and discuss their application for genetic manipulations, including genome engineering and gene silencing.