The phylum Actinobacteria encompasses Gram-positive bacteria with a high DNA G+C. In soils, they present multiple lifestyles: rhizosphere saprophytes, endophytes, facultative symbionts and obligate phytopathogens. They play either beneficial or adverse effects towards plants. Numerous studies focused on their taxonomy and preservation. Also, adequate strategies were developed to enable their isolation. Phenotypically, the Actinobacteria is one of the most diverse phyla within bacteria. Their presence was once monitored by classical culture dependent techniques and phenetic characterization. Development of culture independent methods including chemotaxonomic and genomic approaches such as 16S rRNA gene analysis allowed to detect the so called unculturable bacteria. Major works on their diversity combine culture dependant and independant techniques. This chapter focuses on the phenetic and genetic diversity of Actinobacteria in plant-soil systems. It begins with some knowledge of their biology and their taxonomy, followed by a brief overview of the methods that have facilitated advances in the understanding of their diversity. It also discusses diversity of ecologically important plant associated Actinobacteria (rhizosphere and phyllosphere colonizers, endophytes, symbionts and phytopathogens).