The family Lemnaceae is divided into 2 subfamilies, Lemnoideae with the genera Spirodela and Lemna — the latter with 2 subgenera, Lemna and Staurogeton — and Wolffioideae with the genera Wolffia, Wolffiopsis, Wolffiella, and Pseudowolffia, the last two being new and based on former infrageneric taxa. All these taxa are described, and their types indicated; keys are provided for their distinction. Recognized species names are listed with their synonyms, a few being newly reduced. There are 5 new specific combinations. A list of invalid and dubious names is added. The excluded names are listed separately. An explanatory glossary is given of the terminology used in the descriptions, and is illustrated by some diagrams (fig. 1).