RNAs are exported from the nucleus to the cytoplasm, where they undergo translation and produce proteins needed for the cellular life cycle. Some mRNAs are targeted by different RNA decay mechanisms and thereby undergo degradation. The 5'-->3' degradation machinery localizes to cytoplasmic complexes termed P bodies (PBs). They function in RNA turnover, translational repression, RNA-mediated silencing, and RNA storage. A quantitative live-cell imaging approach to study the dynamic aspects of PB trafficking in the cytoplasm revealed that PB movements are rather confined and dependent on an existing microtubule network. Microtubule depolymerization led to a drastic decrease in PB mobility, as well as a release of regulation on PB assembly and a dramatic increase in PB numbers. The different aspects of PB trafficking and encounters with mRNA molecules in the cytoplasm are discussed.