This study examines the underlying system of BF (Back-Formation) in English. After the synchronic relevance of BF is proved, we present three hypotheses: first, BF as a backward application of a word formation rule, second, BF as a process based on a lexical redundancy rule, and third, BF as a type of conversion. Examining each hypothesis in light of the formal and semantic properties of BF leads us to opt for the third hypothesis, according to which the verb laseV is formed as follows: laserN is converted into laserV, from which the category-marker eris deleted. The deletion is treated as a supplementary operation devoid of derivational significance.