Chapter 1 outlines the legal process of incarceration and the demographics of Britain’s debtors’ prisons, drawing on case studies from London, Edinburgh and Lancaster. It reveals that incarceration for debt was a routine practice rather than an exceptional experience. Britain was home to a substantial population of people living on a thin line between competency and failure. Middling people were especially vulnerable. A middling man in eighteenth-century England had a one in four chance of going to prison during his lifetime. Imprisonment affected their households, neighbourhoods and business networks, so that the impact of incarceration rippled through middling communities. The chapter argues that middling vulnerability was shaped by the legal system, which failed to protect individuals with middle levels of wealth and middle-size debts. While processes of bankruptcy and petty debt courts provided a means of negotiating the largest and the pettiest debts, the population of tradespeople with middle-sized debts, as both creditors and debtors, were underserved by the legal process. Arrest became the most expedient means for creditors to enforce simple debts.