Filamentous cyanobacteria are important primary producers and N 2 fixers in many terrestrial environments. As reduced nitrogen is often limiting, some thalloid liverworts (Marchantiophyta), hornworts (Anthocerophyta), the water fern Azolla (Salviniales), cycads (Cycadophyta), and the angiosperm Gunnera (Gunnerales) have evolved the ability to establish stable and structurally welldefined symbioses with N 2 -fixing cyanobacteria. Also a wide diversity of lichenforming fungi have cyanobacteria as photosynthetic symbionts or as N 2 -fixing symbionts. Cyanolichen symbioses have evolved independently in different fungal lineages, and evolution has often resulted in convergent morphologies in distantly related groups. DNA techniques have provided a wealth of new information on the diversity of symbiotic cyanobacteria and their hosts. The fact that many plants and fungi engage in many different symbioses simultaneously underlines the probable significance of diffuse evolutionary relationships between different symbiotic systems, including cyanobacterial and mycorrhizal associations. This review introduces the reader to recent research on symbiotic cyanobacteria in terrestrial ecosystems and shortly describes the astonishing range of diversity in these ecologically important associations.