Preprocessing is a critical step in preparing neuroimaging data for analysis, but current practices are often ad hoc and poorly documented. WG3 aims to standardize and rationalize preprocessing procedures to enhance transparency, reproducibility, and interoperability. Focusing initially on EEG and MEG, this Working Group will define best practices and documentation standards for preprocessing pipelines, drawing on successful examples from MRI. The outcome will be clearer, more reusable pipelines that support consistent and trustworthy analyses across research sites.