The fair, competent, effective, and predictable application of laws that enhance, rather than undermine, social accountability and fundamental human rights is a core function of the state and part of its social contract with citizens. Ensuring that a government upholds the rule of law requires accountability and enforcement. But a republican structure of checks and balances can be unresponsive or resistant to scrutiny when elites across the branches of government are indifferent to, or collude in, chronic problems in the justice system. Active non-governmental organisations (NGOs) are therefore crucial to the effective application of the rule of law due to their independence and direct contact with the victims of arbitrary treatment. This chapter examines ways how NGOs, both international and local, can fulfil this important watchdog function and contribute to strengthening rule of law by working strategically with one another, with international partners and with sympathetic state actors. It is centred on a case study of how the Open Society Institute and its Justice Initiative (OSJI) and a network of Brazilian NGOs developed a campaign to reduce the excessive use of pretrial detention and were able to reform laws, policies and practices.