This thesis is based on the following publications, which are referred to in the text by their Roman numerals:
[I] Halonen P & Puolasmaa A (1995) The lichen genus Usnea in eastern Fennoscandia. I. Usnea hirta. Ann Bot Fenn 32: 127–135.
[II] Halonen P (1997) The lichen genus Usnea in eastern Fennoscandia. II. Usnea longissima. Graphis Scripta 8: 51–56.
[III] Halonen P, Myllys L, Ahti T & Petrova OV (1999) The lichen genus Usnea in East Fennoscandia. III. The shrubby species. Ann Bot Fenn 36: 235–256.
[IV] Halonen P, Clerc P, Goward T, Brodo IM & Wulff K (1998) Synopsis of the genus Usnea (lichenized Ascomycetes) in British Columbia, Canada. Bryologist 101: 36–60.
[V] Halonen P (2000) Usnea pacificana, sp. nov. and U. wasmuthii (Lichenized Ascomycetes) in Pacific North America. Bryologist, in press.