In many applications, it is not possible to measure distance to the target by establishing a physical contact. Typical examples include measuring presence of objects on a conveyor belt in logistic centers, or ensuring safety distances around moving robot arms. Other possible options for contactless distance measurement are eddy currents, ultrasounds and light. Light distance and ranging (LIDAR) systems use the time taken by the light to travel back and forth to an object to measure the distance to this object. A LIDAR system can be built with either a high-speed ADC or a time-to-digital converter (TDC). TDCs can be a timer in a microcontroller/microprocessor (MCU/MPU), dedicated logic on a field-programmable gate array (FPGA), or more simply, a dedicated TDC integrated circuit (IC). This article is an introduction to designing systems based on TDCs and covers optical and analog front-end considerations, as well as basic signal processing for TDCs.