There are no legal institutions other than pignus and hypotheca (i.e. mortgage) where the formative effect of legal practice can be so clearly observed. Security and Credit in Roman Law outlines the legal history of these institutions in terms of an iterative relationship between transactional lawyers drafting legal transactions and Roman jurisprudence deploying its analytical skills in order to accommodate new transactional practices into the Roman legal system. The evolution of the Roman law of real security, well known through the legal sources (Justinian's Digest and Code), is reconstructed, while matching it with actual banking practices, in particular the secured lending transactions documented in the archive of the Sulpicii. In the late classical period the imperial chancery increasingly interfered with it in order to provide a considerable degree of protection to debtors. The (largely but certainly not completely) spontaneous evolution of Roman law produced a law of secured transactions which was highly sophisticated and versatile, allowing non-possessory security, multiple charges, pledges of receivables, antichretic pledges, and even floating charges over a dynamic fund of assets. Since legal systems often adapt in reaction to impulses from their economic environment, the complexity of the Roman law of real security indicates that pignus and hypotheca did play a significant role in the Roman economy. It will be shown that this role was generally a positive one. Its main weaknesses were lack of publicity and the presence of fiscal charges: even these weaknesses did not undermine the effectiveness of secured transactions.