Internationally, learning science through investigation is promoted as a preferred pedagogical approach. Research presented takes a view that such learning depends on how teachers understand science investigation. Teachers‘ understanding of science investigation was an aspect of an interpretive case study of the phenomenon of science investigation exploring the links between learning, motivation and assessment in year 11 science. Data were collected through a population survey of year 11 science teachers (n=165) in the greater Wellington region through a postal questionnaire (response rate 61%). In addition, all year 11 science teachers in a typical coeducational, middle size, urban secondary school were interviewed (n=10). Findings suggest that science investigation that best supported student learning was understood to include experiments, scientific method, and fair testing, and that few teachers demonstrated understanding of an open-ended science investigation. Teachers‘ responses indicated the influence of assessment requirements of a linear and sequential fair testing type of investigation. This has implications for teaching investigation as required by the curriculum, and student learning for assessment rather than an understanding of the nature of scientific investigation.