The cytoplasmic destination of transcripts is thought to be determined by the cis-acting sequences that usually lie in its 3’TR and act as zipcodes, by the trans-acting factors that interpret the localisation signals and by the type of motors that are engaged to transport RNP (Ribonucleoprotein) particles. The molecular motors Dynein and Kinesin transport their RNA cargoes over long distances towards the minus or plus ends of microtubule tracks respectively, whereas myosin mediates short-distance transport along actin microfilaments. Although many cargoes undergo net displacement and achieve asymmetric intracellular distribution over time, their motion is not always highly unidirectional and often interrupted by pauses and switches in direction. In Chapter 3 of this thesis, we investigate in detail the role of motors and their cofactors in bidirectional RNA motility in Drosophila embryos. We show that Kinesin I or Kinesin II are not involved in the reverse motion of dynein-driven RNA cargoes and we suggest that dynein moves bidirectionally in vivo. Furthermore, we demonstrate that dynactin, dynein’s accessory factor, is required to reduce backwards motility. We suggest that dynactin mediates suppression of reversals by stabilising dynein on the microtubule or by assembling and coordinating multiple dyneins together. In Chapter 4, we explore novel genome-wide bioinformatics approaches for the identification of localisation signals and localising transcripts in Drosophila embryos and oocytes and we show that the method successfully predicts localisation signals in transposable elements and in the 3’UTR of endogenous Drosophila genes. Finally, in Chapter 5, we address the suitability of various Drosophila cell lines for the establishment of an RNA localisation assay, with the aim of using the assay for a genome-wide RNAi screen for novel RNA localisation factors.