Introduction

Although others are cautious, many in the evaluation community see ‘theory- based evaluation’ (TBE) as an attractive new set of approaches for tackling complexity evaluations. In fact, this ‘new’ idea has been around a long time, with its roots in the early 20th century in analytic philosophy which helped shape design and delivery in engineering and technology through ‘conceptual design’. This later contributed to ideas about ‘programme theory’ in social and behavioural sciences, going on to influence evaluation thinking from the 1970s (Carol Weiss, 1972; Huey Chen and Peter Rossi, 1980).

As a cohesive evaluation approach, TBE came to prominence with Chen's Theory- driven Evaluations (1990) and 20 years later its use could be cited in nearly 50 evaluations, mainly from the US (Coryn et al, 2010). Despite an important formative contribution in Europe from Ray Pawson and Nick Tilley in Realistic Evaluation (1997) the practice outside the US was slower to take off. Much evaluation activity in Europe was directly funded or co- funded by governmental and other public bodies which, until recently, often discouraged use of these approaches.

Opportunities for the use of TBE are rising. In the UK, for example, government has since 2020 encouraged the use of such approaches for some of the complex evaluation circumstances which characterise many public policy and programme interventions. Yet cautions continue in many parts of government and for many other funders, with TBE use remaining embryonic, especially for public bodies. It seems that while the methodological foundations are in place, their expression remains under- utilised and under- developed. This chapter looks at their considerable potential for purposeful impact evaluation.