Objectives: To summarize the available evidence on the diagnostic performance for invasive aspergillosis (IA) in non-hematological, non-solid organ transplantation critically ill patients of the following: (i) existing definitions of IA (developed either for classical immunocompromised populations or for non-immunocompromised critically ill patients); (ii) laboratory tests; (iii) radiology tests. Methods: A systematic review was performed by evaluating studies assessing the diagnostic performance for IA of a definition/s and/or laboratory/radiology test/s vs. a reference standard (histology) or a reference definition. Results: Sufficient data for evaluating the performance of existing definitions and laboratory tests for the diagnosis of IA in critically ill patients is available only for invasive pulmonary aspergillosis. Against histology/autopsy as reference, the AspICU definition showed a promising diagnostic performance but based on small samples and applicable only to patients with positive respiratory cultures. Studies on laboratory tests consistently indicated a better diagnostic performance of bronchoalveolar lavage fluid (BALF) galactomannan (GM) than serum GM, and a suboptimal specificity of BALF and serum (1,3)-β-D-glucan. Conclusions: Evidence stemming from this systematic review will guide the discussion for defining invasive aspergillosis within the FUNDICU project. The project aims to develop a standard set of definitions for invasive fungal diseases in critically ill, adult patients.