We present the HoloDark technique which has recently been invented and allows one to map strain in two dimensions in layers and devices with nanometer resolution, high precision and large field of view. The technique is based on electron holography and is applicable to all standard focused-ion beam FIB prepared crystalline samples. We show a panorama of typical results obtained in SiGe stacks, ion implanted silicon, strained silicon channel nMOS and pMOS type transistors and in the challenging case of strained silicon FinFETs, In such materials and structures, the HoloDark technique, although still perfectible, appears as the only technique able to provide reliable and extended data against which simulations can be calibrated.