This is a still provisional guide to parmelioid lichens occurring in northern Thailand, which will be tested in the field during an excursion on the occasion of the 7th Congress of the International Association of Lichenology (IAL7). The species included here are primarily based on specimens seen by the first and last two authors in the herbaria BM, CMU, TNS, and RAMK. This research was undertaken between 1990-2004 by Wolseley, Aguirre-Hudson, and Saipunkaew, and first presented at an IAL meeting in Kunming in 2002. Since that time there have been many changes in Parmeliaceae concepts as well as further research on this group in Thailand. We have also included some foliose 'cetrarioid' species such as Cetreliopsis, Cetrelia, and Nephromopsis (the latter 2 still under study), and some species reported from northern Thailand in the recent literature, which brings the number of infrageneric taxa to 99.Following testing of the key and discussion at IAL7, we would like to enlarge the key to include further records.The main aim of this key is to provide simple descriptions of species and key characters to enable identification of this conspicuous group of lichens in SE Asia by both professional and nonprofessional lichenologists. Chemistry that can be elucidated using spot tests is included in the key. Differences between similar and/or closely related species in the region are mentioned in the text.The key was generated in Italy, at the University of Trieste, using the FRIDA software. It has two query interfaces: 1) an illustrated dichotomous interface, 2) a multi-entry query interface. The dichotomous key is available in different versions for different media: a) internet, b) CD-Rom, c) printable (pdf), d) for mobile devices (PDAs, iPhone, iPad, and iPod touch).