Albeit restricted to a subset of cell types, polarized cell tip growth is a fundamental development mechanism in the plant kingdom from algae to angiosperms. We are focusing in this chapter on the current understanding of such a growth mechanism in mosses, withPhyscomitrella patensas a model. Spore germination and the first stage of the haploid developmental phase, the protonemata, rely completely on tip growth for their development. Simultaneously to this purely developmental aspect, protonemal tip cells integrate environmental cues (e.g. light, gravity) and respond in a coordinated and polar manner. Different physiological and pharmacological studies have implicated directly and indirectly the actin cytoskeleton as a central component in both aspects of tip growth. With the recent development of the reverse genetic tool kit inP. patens, the role of the actin cytoskeleton in regulating tip growth can now be finely dissected. So far, genetic disruption of four components of the conserved eukaryotic actin cytoskeleton regulation machinery, Arp2/3 complex and its regulatory WAVE complex, profilins and ADF/cofilin confirmed a direct implication of the actin cytoskeleton in polar tip growth, allowing the establishment of a functional model of cell tip elongation inP. patens.