The rise of the accountability movement has led to the proliferation of performance management where voluntary organisations are under increasing pressures to measure the outcomes of their social services, in an effort to maintain accountability and to facilitate learning and improvement. However, there is little empirical evidence on whether and how performance management works to achieve accountability and improvement in practice. The study undertakes a realist evaluation to examine how and why performance management works (or not) to achieve accountability and improvement among voluntary organisations, through the use of performance information within the context of the voluntary sector in Singapore. The National Council of Social Service has disseminated a national performance management framework, known as "Outcome Management" to the voluntary welfare organisations for measuring the outcomes of the social services they deliver. The theory of how and why Outcome Management works to achieve accountability and improvement, is often not explicitly stated but simply assumed by the actors involved its design and implementation. To unpack the "black box", initial programme theories of how and why Outcome Management is supposed to work are tested against the actual programme theories of how and why Outcome Management actually works in the seven identified case studies. Findings showed that the performance information generated has been largely used for meeting targets rather than for improving services. Outcome Management, in its current implementation, is not capable of testing the theory of change underlying the social services. And such testing forms the logical basis for learning and improvement. Also, Outcome Management is found to be more suitable for measuring the outcomes of "programmatic interventions" as compared to "single practitioner-driven interventions". Across most case studies, the emergence of poorly-defined outcomes, undefined or missing programme theory of change and the lack of standardisation in the use of measurement tools and administering procedures has undermined the success of Outcome Management. In light of the empirical findings, revisions are made to the initial programme theories, leading to refined programme theories of how and why Outcome Management could work better, for whom and under what conditions to achieve accountability and improvement.