Bacteria of the genus Brachyspira are intestinal spirochaetes that can cause diarrhoea and mortality in pigs. Laboratory diagnostics of Brachyspira species is essential for confirming clinical diagnosis, for providing data for optimal treatment and for surveillance of the bacteria in individual animals or herds. The aims of the present thesis were to evaluate presently used, and develop new, laboratory diagnostic techniques for Brachyspira species, to describe the pathogenic and epidemiological features and antimicrobial susceptibility of selected Brachyspira isolates, and to evaluate DNA-based epidemiological and phylogenetic tools. In study I, seeded samples were sent to laboratories in northern Europe to assess their ability to detect and identify Brachyspira species and test antimicrobial susceptibility. In study II, a PCR system was set up and compared with traditional culture and biochemical tests; and in study III, well-described Brachyspira isolates were analysed by five molecular typing methods. In the last two studies Brachyspira isolates from animals other than pigs were included. An atypical isolate detected in study II was further characterised together with identical or similar isolates (study IV) and a new genetic typing method for the entire genus of Brachyspira species was tested and evaluated (study V). Overall, the results indicated that laboratory diagnostics of Brachyspira species is difficult. Of the tested genetic typing methods, the nox gene and MLST sequence analyses showed the highest taxonomic resolutions, and randomly amplified polymorphic DNA showed the highest discriminatory power for the isolates studied. A high genetic variability was observed within the Brachyspira genus. This genetic variability could make it difficult to rely solely on DNA-based methods for detection and identification of brachyspiras. A new unique group of isolates pathogenic to pigs was detectable only by the combined use of culture, biochemical tests and PCR. The provisional name "B. suanatina" was suggested for this type of isolates, of which some were found in mallards. By MLST data analysed by e-BURST, a close evolutionary relationship was identified for "B. suanatina" and B. hyodysenteriae isolates recovered from pigs and mallards. In conclusion, these studies show that the most reliable method for laboratory diagnostics of Brachyspira species is culture and biochemical tests, used together with at least one DNA-based method.