L-forms are "cell wall-deficient" bacteria which are able to grow as spheroplasts or protoplasts. They can be differentiated into four types depending on their ability to revert to the parental, cell-walled form and to the extent of their cell-wall modification. L-forms are significant in modern science because of their contributions to an improved understanding of principal questions and their interactions with eukaryotes. This review particularly focuses on research using stable protoplast-type L-forms which have contributed to a better understanding of the structural and functional organisation of the cytoplasmic membrane and of cell division. These L-forms, which have only a single surrounding bilayer membrane, also represent a unique expression system for production of recombinant proteins. A large proportion of L-form publications concern their putative role in human disease and its therapy, a topic which is discussed briefly. L-forms have also been used to form intracellular associations with plant cells and have been shown to elicit induced disease resistance offering a novel method for plant protection. The recent decline in active research on L-forms is a concern as knowledge and experience, as well as unique L-form strains which have been maintained for decades, are being lost.