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The increasing performance demanded by emerging wireless communication standards motivates the development of various techniques devoted to improving the efficiency of power amplifiers (PA) because this is one of the most power-demanding blocks in RF transceivers. Power-amplifier efficiency is proportional to the ratio of the average voltage delivered by the PA to the voltage level of the PA's power supply. Efficiency is affected by the peak-to-average ratio of the transmitted signal. The envelope tracking modulator maximizes this ratio, correlating the PA's power supply with the envelope of its output signal. Efficient modulators must satisfy certain critical conditions: i) it must be very agile to track the amplitude variations of PA's output voltage; ii) it must reduce the timing mismatch between the PA modulator's supply and PA output waveform envelope to optimize power efficiency and avoid PA saturation, and iii) the envelope tracking modulator must be highly power efficient. This paper reviews several relevant envelope tracking techniques. Hybrid modulators consisting of switching regulators and linear amplifiers have become mainstream envelope tracking systems for wideband applications, in which linear amplifiers complement the functionality of highly efficient but narrow bandwidth switching modulators. Replacements for linear amplifiers include a combination of power-efficient ADC and DACs that provide very agile feedback, increasing the system's slew rate, which allows the modulator to track faster envelope signals. Multi-level switching is another relevant approach utilizing multiple switching voltages to reduce current ripples and enable the use of wider bandwidth switching regulators with high power efficiency. The use of multiple inductors is another interesting approach. Multi-phase switching techniques utilize multiple switching stages in a time-interleaved manner to extend the switching modulator's bandwidth. A slow buck converter can be combined with a fast buck converter and optimized for different switching frequencies; this architecture covers the signal envelope's low- and high-frequency components. The approaches mentioned use switching modulators with analog feedback controllers (Pulse-width modulation [PWM] or hysteretic). However, an alternative approach is prediction-based digital feedforward control. This tutorial discusses all of these approaches.