We are developing Ultra-Fast Silicon Detectors UFSD, providing experiments with high-precision 4D capability. The principle is based on fast collection time, i.e. thin sensors, and requires sensors with internal gain. I will discuss applications in a wide range of fields, and expected performance based on estimation of the signal-to-noise ratio. I will report on electrical tests and TCT charge collection data with so-called low-gain avalanche detectors (LGAD). I will compare the bias dependence of the pulse shapes of traditional sensors and of LGAD sensors with different dopant density of the diffusion layer, and extract the internal gain.