Pre-aspiration can be defi ned as a period of glottal friction, [h], which is found in the sequences of sonorants and phonetically voiceless obstruents, as in map [ma h p h ] (e.g. in Welsh English, Hejná 2015). This chapter problematises the default approach to pre-aspiration shown in a number of studies, which assume that unless pre-aspiration applies obligatorily it is subject only to phonetic constraints rather than, at least potentially, both phonetic and phonological constraints. As a result, instances where pre-aspiration reaches the duration of 0 ms, i.e. where it does not apply (mat [mat s ]), are typically included in the analyses of its phonetic conditioning. This can be problematic in cases where zero values refl ect a variable phonological rule rather than the output of solely phonetic constraints as such an approach may obscure our understanding of the constraints on pre-aspiration. [I]t may be time to consider the zero.