Researchers' notions of research quality depend on their field of research. Previous studies have shown that field-specific assessment criteria exist but could explain neither why these specific criteria and not others exist, nor how criteria are used in specific assessment situations. To give initial answers to these questions, formal assessment situations (reviews of publications) and informal assessment situations (seminar and conference discussions) in two specialties (plant biology and medieval history) were analysed through observations and qualitative content analysis. The empirical findings reveal a large variety of quality criteria used by researchers, which cannot be subsumed under conventional notions of originality, reliability, or validity. The identified field-specific quality criteria can be explained by differences between the two fields' research practices and between the epistemic properties of the two fields.