Invasive mould diseases, particularly aspergillosis, are important causes of morbidity and mortality in allogeneic stem cell transplant recipients. Mould spores are ubiquitous in the environment. Guidelines established by the Centers for Disease Control (CDC) and other authoritative organizations focus on approaches to reduce exposure to mould spores. These recommendations include avoidance of areas and activities expected to result in high levels of mould spores (e.g., construction, gardening) and use of specially designed units (protected environments) where additional standards (e.g., HEPA-filtered rooms) are in place to minimize mould exposure. These recommendations are based on consensus criteria and limited clinical data largely derived from single-center retrospective studies. In addition, highly immunocompromised stem cell transplant recipients are commonly managed as outpatients, where engineering standards of the inpatient protected environment are not feasible. In the absence of an outbreak with an identified environmental source (e.g., a contaminated air vent), it is not possible to reliably distinguish community-acquired from nosocomial aspergillosis. Adherence to infection control guidelines, acknowledging their limitations, combined with evidence-based targeted antifungal prophylaxis for the highest risk transplant recipients, is likely to be the most effective approach to prevent invasive mould diseases.