Alien flora of the Czech Republic is presented. In Appendix 1, 1378 alien taxa (33.4% of the total flora) are listed with information on the taxonomic position, origin, invasive status (casual, naturalized, invasive; a new category post-invasive is introduced), time of immigration (archaeophytes vs. neophytes), habitat type invaded (natural, seminatural, human-made), vegetation invaded (expressed as occurence in phytosociological alliances), mode of introduction into the country (accidental, deliberate), and date of the first record. Number of phytogeographical as well as biological and ecological attributes were compiled for each species in the database; its structure is presented in Appendix 2 as a suggestion for similar work elsewhere. Czech alien flora consists of 24.1% of taxa which arrived before 1500 (archaeophytes) and 75.9% neophytes. There are 891 casuals, 397 naturalized and 90 invasive species. Of introduced neophytes, 21.9% became naturalized, and 6.6% invasive. Hybrids contribute with 13.3% to the total number of aliens, and the hybridization is more frequent in archaeophytes (18.7%) than in neophytes (11.7%). If the 184 hybrids are excluded from the total number of aliens, there are 270 archaeophytes and 924 neophytes in the Czech flora, i.e. total of 1195 taxa. Accidental arrivals account for 53.4% of all taxa and deliberate introduction for 46.6%; the ratio is reversed for neophytes considered separately (45.5 vs. 54.5%). Majority of aliens (62.8%) are confined to human- made habitats, 11.0% were recorded exclusively in natural or seminatural habitats, and 26.2% occur in both types of habitat. Archaeophytes and neophytes occur in 66 and 83 alliances, respectively, of the phytosociological system. Flora is further analysed with respect to origin, life histories, life forms and strategies. Only 310 species (22.4% of the total number of all alien taxa) are common or locally abundant; others are rare, based on a single locality or no longer present. The following 19 taxa are reported as new for the Czech alien flora: Agrostis scabra, Alhagi pseudalhagi, Allium atropurpureum, Bromus hordeaceus subsp. pseudothominii, Carduus tenuiflorus, Centaurea ×gerstlaueri, Centaurea nigra ×phrygia, Cerastium ×maureri, Gilia capitata, Helianthus strumosus, Hieracium pannosum, Hordeum leporinum, Oenothera coronifera, Papaver atlanticum subsp. mesatlanticum, Parietaria pennsylvanica, Polypogon fugax, Rodgersia aesculifolia, Sedum pallidum var. bithynicum, Sedum stoloniferum; these represent results of our own field research as well as of herbaria search, and unpublished data from colleagues. Other 44 taxa are reported as escaping from cultivation for the first time. Twenty two archaeophytes are listed in the Red List of the Czech flora.