The question 'What makes someone the parent of a child?' is at once straightforward and complex. Straightforward because we often have what we feel to be a 'common sense' or 'intuitive' response. This might be with respect to individual parent-child relations -'Z and Y are X's parents' -or it might relate to a more generalized normative standard -'the woman who gives birth to you is your mother'. However, if we collected a number of these 'common sense' or 'intuitive' responses, we are likely to find variation and contradictions within, signalling that our notions about parenthood are rather more complex than we might first envisage. In this chapter, I set about exploring this complexity through the lens of legal parenthood in the UK. It should be noted from the outset that I am not suggesting law to be somehow representative of how people understand parenthood: this is an empirical question far beyond the scope of this short chapter. Instead, what I am interested in are the various grounds upon which a person may be regarded as a legal parent at the moment of a child's birth in the UK. When we investigate this closely, we see that while these grounds have arguably shifted and expanded over recent decades, the notion that a child has two 'real' parents has remained constant. This is reflected in the strict two-parent model for legal parenthood (see also Lotz, this volume).Although legal parenthood has primarily developed through the common law, specific statutory provisions have been enacted in response to a number of assisted reproduction techniques, namely donor insemination, IVF using donated gametes and certain surrogacy arrangements. While clearly these provisions affect a relatively small proportion of births in the UK, the extent to which assisted reproduction generates controversy and captures the social imagination renders related legislation highly significant and symbolic.Moreover, Part 2 of the Human Fertilisation and Embryology Act 2008 ('the 2008 Act') provides the most recent formal statement and codification of legal parenthood in the jurisdiction. That this codification reaffirms the strict two-parent model for legal parenthood is important, not least because the parenthood provisions apply to reproductive techniques which very directly dismantle the seemingly axiomatic two-parent connections of sexual reproduction which conflate bio-genetic ties with socio-legal roles and responsibility in most, if not all, Western societies (Boyd 2007;Diduck 2007;Dolgin 1997;Franklin 1997;Strathern 1992). What purpose does reaffirming the two-parent normative model serve and what does it say about our understandings of parenthood and family more generally? What tensions does it present for how legal parenthood is determined in the context of sexual reproduction where bio-genetic ties are increasingly determinative, especially for fatherhood? Did the law-makers involved really engage with the question of who counts as a parent and on what basis? Or were their deliberations primarily informed by 'common sense' a...