This paper examines the influence of European Union law on corporate taxation, particularly concerning the taxation of profits and the deduction of losses. It highlights the crucial role played by the Court of Justice of the European Union (CJEU) in addressing discriminatory national tax measures that restrict EU Treaty freedoms, such as freedom of establishment and movement of capital. The study reviews landmark CJEU rulings on equal tax treatment for permanent establishments and subsidiaries, the deductibility of cross-border losses, and group taxation within the EU. The Court has clarified that Member States must not discriminate based on company location and must respect legal certainty and proportionality. While EU law does not harmonize direct taxation, the CJEU ensures negative harmonization by invalidating incompatible national measures. Member States must adapt their tax regimes accordingly, although variations in implementation persist. The paper also addresses anti-abuse rules and final loss deductibility in cross-border contexts.