Abstract: We recently proposed a surface-mounted structural health monitoring (SHM) scheme based on commercial, low-cost inertial MEMS sensors. While such commercial-off-the-shelf sensors are not very accurate, their low cost and negligible weight allow them to be deployed in dense arrays, possibly overcoming inaccuracy through redundancy. Taking the sensor characteristics into account, the development of a MEMS-based SHM method for lightweight structures like thin plates, was tackled from two different viewpoints: sensor accuracy verification, and optimal sensor placement. To assess the accuracy, a preliminary investigation was run on standard composite specimens for delamination testing, adopting a single MEMS three-axis accelerometer. A theoretical interpretation of the results, based on beam bending theory, showed the ability of the system to provide a one-to-one relationship between the crack length and the sensor output. Concerning the placement of the sensors, an approach for their optimal deployment over thin structures was developed, using a topology optimization-like formulation. Such formulation is able to search for the optimal layout of the network, by maximizing the sensitivity of the overall sensors output to a damage possibly located anywhere. In this work, accounting for the characteristic sizes of a structural element and of the MEMS package, which might differ by orders of magnitude, we also introduce a multi-scale (actually, two-scale) approach to sensor deployment. It is shown that, no matter what the location, size, and shape of the damaged area are, a trivial array of evenly spaced sensors does not represent the optimal solution to monitor the structural health.