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Diagnostic functions will be making the difference in photovoltaic systems. The need of a daily, monthly and yearly quantification of the energy produced by a plant is a feature required by the customers for comparing the actual performances of their system with the expected values in order to quantify the real return on investment time. In some cases, monitoring facilities are offered by the producers of the power processing system but some companies also offer power production monitoring as a service. The power and energy monitoring is a first step towards a more complete diagnosis of the photovoltaic modules and prognosis about the expected lifetime, which would be welcome by the market. The module's productivity is a too coarse information for understanding the problems affecting the modules and for performing the right repairing. Thus, new and even more advanced diagnostic algorithms must be developed in order to monitor the status of the cells and to predict failures before they lead to a significant reduction on the energy produced by the module. Such failures might be not only due to cells malfunctioning, but also to faults of the bypass diode the module is equipped with. This might happen due to repeated shadowing or to lightning. In this paper the techniques for photovoltaic modules diagnosis proposed in the recent literature are overviewed. The main features of some commercial products are also given. Some elements to be used as a starting point for the development of algorithms for diagnostic and prognostic purposes at module level are proposed.