MNE-CPP is an open-source, cross-platform C++ framework for real-time and offline processing of MEG, EEG, and related neurophysiological data. Developed within the MNE ecosystem, it provides a modular and high-performance software platform for electrophysiological data acquisition, raw-data inspection, sensor- and source-level analysis, and interactive 3D visualization. The framework integrates reusable C++ libraries with user-facing applications, including MNE Scan for real-time acquisition and processing, MNE Analyze for sensor- and source-level analysis of recorded data, MNE Browse for interactive inspection of FIFF raw data, and MNE Inspect for 3D visualization of anatomical and functional neuroimaging data. Complementary command-line tools support forward and inverse modeling, BEM processing, anonymization, conversion, and real-time streaming workflows. Designed for both scientific research and practical deployment, MNE-CPP serves as a foundation for custom neuroscience software as well as complete user-facing neurotechnology applications. The project is community-driven, BSD-3-Clause licensed, and maintained as part of the public mne-tools open-source ecosystem.