This thesis consists of a study of English and US corporate finance law and, in particular, the law in relation to hybrid financial instruments. Hybrids of financial instrument presents a mix of equity and debt characteristics. Therefore this thesis excludes from examination all the derivative instruments, while it focuses on two main types of hybrid security, in relation to their relevance to the situation studied: preference shares and convertible bonds. Despite a clear distinction in law between equity and debt, the development of sophisticated hybrid financial instruments has forced regulators to look beyond the legal form of an instrument to its practical substance. As observable in practice, the increase in financial innovation reflects the necessity of the parties to allocate control and cash-flow rights in a way that diverges from the classic allocation resulting from equity and debt.