In this chapter we will consider some of the fundamental concepts associated with analog modulation. Communication of analog signals from one location to another is accomplished by using either a wire channel or a radio channel. The source signals, such as voice, pictures, and in general baseband signals are not always suitable for direct transmission over a given channel. These signals are first converted by an input transducer into an electrical waveform referred to as the baseband signal or the message signal. The spectral contents of the baseband signals are located in the low-frequency region. The wire channels have a low-pass transfer function and can be used for transmitting signals that have a bandwidth less than the channel bandwidth. The radio channels have a band-pass characteristic. Low-pass signals can be transmitted through radio channels by using a modulator.