It is common ground that a bill of constitutional rights regulates the relationship between individual and the state. Fundamental rights create negative obligations for the state – duties not to interfere with life, liberty, conscience, speech, privacy etc. To what extent, if at all, though, do fundamental rights also have a bearing on relations between individuals? Can they also obligate the state to actively promote liberty in society? Are they focused merely on state abuse but silent on social inequality – or can they be mobilized as vehicles for social justice? Undeniably, constitutionally-sensitive conflicts routinely arise, e.g., between capital and labour; between financial service providers and vulnerable consumers; between vulnerable groups themselves, and so on. However, the “constitutionalisation” of private law relationships has not always been seen as an unmixed blessing. A first concern is that the expansion of substantive constitutional norms beyond the negative basic liberties eliminates the domain of private law and private autonomy that is an essential characteristic of liberal societies. A second concern is that constitutionalisation transfers substantial regulatory authority from democratic legislatures to constitutional courts. This chapter argues that that neither of these two concerns necessarily arises in connection with the doctrine of (indirect) horizontal effect.