The deployment of PEM fuel cell systems is becoming an increasingly pivotal aspect of the electrification of the transport sector, particularly in the context of heavy-duty vehicles. One of the principal constraints to market penetration is durability of the fuel cell which hardly meets the expected targets set by the vehicle manufacturers and regulatory bodies. Over the years, researchers and companies have faced the challenge of developing reliable diagnostic and condition monitoring tools to prevent early degradation and efficiency losses of fuel cell stack. The diagnostic tools for fuel cell rely usually on model-based, data driven and hybrid approaches. Most of these are mainly developed for stationary and offline applications, with a lack of suitable methods for real-time and vehicle applications. The work presented is divided into two parts: the first part explores the main degradation conditions for a PEMFC and characteristics, advantages, and application limits of the main methodologies for fuel cell diagnostic, while in the second part the features and the development process of an innovative, real-time, and on-board health and condition monitoring system, based on electrochemical impedance spectroscopy (EIS), are presented. The new innovative tool allows to detect, identify and isolate degradation, faults and non-optimal conditions. The computational performance and reliability of the diagnostic tool are tested and validated through experimental tests carried out in the laboratory on single cell and short fuel cell stack over a wide range of operating conditions and under specific sub-optimal/fault states such as drying, flooding and reactants starvation. The condition and health of PEMFC are estimated using specific health indicators for the most common root causes of faults such as drying, flooding, catalyst poisoning and anode/cathode starvation.