In future smart grids there is an increased need for smart diagnostics. Diagnostics are SMART if they are Specific, Meaningful, Actual, Reliable and Timely. These properties are explained in this paper, together with an explanation of the need for smart diagnostics in any grid, but particularly future smart grids. An example is given for a smart diagnostic for complete medium voltage power cable connections. This Smart Cable Guard monitors the condition of MV power cables continuously and also provides for continuous access to the associate risk levels. As this system is applicable in any grid, some examples are given.