We are pleased to dedicate this issue of The Lichenologist to Teuvo ‘Ted’ Ahti on the occasion of his 90th birthday. Teuvo has already been honoured by the publication of a Special Issue^{Reference Stenroos1}, at the time of his 60th birthday, but as his close colleague Irwin ‘Ernie’ Brodo puts it, ‘if anyone deserves two Festschrifts, it would be Teuvo’. All the articles in the present issue are invited contributions from Teuvo's friends and colleagues worldwide, in recognition of his outstanding contributions to lichenology. Many more individuals would have liked to have participated than it has been possible to accommodate within a tight schedule.

In the first contribution in this Special Issue – a Laudatio – Orvo Vitikainen, Irwin Brodo and Trevor Goward share their recollections of Teuvo and his remarkable career. The Laudatio is followed by ten scientific papers which cover a wide range of themes reflecting not only the main lichenological topics Teuvo has dealt with during his career, but also his admirable ability to establish connections and make friends wherever he goes. In the paper by Davydov et al., Umbilicaria ahtii sp. nov., from the U. vellea species complex, is described based on morphological and molecular characters; Randlane et al. provide a comprehensive overview of the red-listed lichens in Estonia; Tibell et al. describe a new lichenicolous species, Calicium ramboldiicola, growing on Ramboldia elabens; Wheeler et al. present a five-locus phylogeny of the family Megasporaceae and, based on this, propose several taxonomic innovations including the description of new genus Antidea; Yakovchenko et al. describe a new species, Candelariella ahtii, based on phenotypic and ITS nrDNA sequence data and provide a worldwide key for all known Candelariella species with 8-spored asci; and Zhurbenko provides a key to 36 species of lichenicolous fungi and lichens known to occur on Rhizoplaca s. lat., including four new species and one variety new to science. Teuvo is perhaps best known as a specialist on the family Cladoniaceae, and three papers are centred on this group. Pino-Bodas et al. examine the phylogenetic relationships in the Cladonia cervicornis group, describing a new lineage C. teuvoana; Mercado-Díaz et al. use phylogenetic and population genomic analyses based on RADseq data to clarify the evolutionary relationships and phylogeography of Cladonia sandstedei in the Caribbean; and McCune et al. explore the use of an anisaldehyde reagent for visualizing usnic acid particularly in the genus Cladonia. Finally, in a contribution from Teuvo's Finnish colleagues, Pykälä et al. present their studies on Verrucaria and describe seven new species from calcareous and siliceous rocks of Finland.

We thank everyone who made this Festschrift possible, including the reviewers of the contributed papers.