Several classes of endogenous small RNAs of 20-26 nucleotides, such as microRNAs and small interfering (si)RNAs, have been described in plants and animals. Arabidopsis currently seems to be an exception because a novel genetically distinguishable small RNA class, termed natural-antisense transcript derived siRNAs, was recently identified. Pathways for the biogenesis and function of each class are already well characterized, enabling their classification on the basis of both size and proteins required for their function. However, each pathway requires proteins belonging to conserved protein families, and recent genetic studies confirmed an expected partial redundancy between members of the Dicer-like (DCL) family. Moreover, several experimental results indicate redundancies among double-strand RNA binding proteins and ARGONAUTE proteins, which probably create a complex network of small RNA pathways.