The next generation of collider detectors will make full use of Particle Flow algorithms, requiring high precision tracking and imaging calorimeters. Featuring a granularity by 2 to 3 orders of magnitude higher than existing devices, imaging calorimetry has been developed during the past 15 years by the CALICE collaboration and is now reaching maturity. The state- of-the-art status and the remaining challenges are reviewed here for all investigated sensor types: silicon diode and scintillator for electromagnetic calorimeters, gaseous with semi-digital readout, and scintillator with SiPM readout for hadronic ones. An overview of recent commissioning and beam test results of large-scale technological prototypes and raw performances such as energy resolution, linearity, and studies exploiting the distinct features of granular calorimeters regarding pattern recognition is presented. Beyond these prototypes, the design of experiments addressing the requirements and potential of imaging calorimetry is discussed.