The aim of the book is to announce audience about the principles of function circuits. The book defines (i) what is a function circuit? (ii) What are types of function circuit (iii) how the types of function circuits are derived from saw tooth and triangular waveforms and (iv) how the derived function circuits are implemented with integrator, comparator, low pass filter, peak detector and sample & hold circuits. The principles of time division function circuits are explained by generating a rectangular pulse using either saw tooth or triangular waveform and averaging the same rectangular pulse with a low pass filter. The principles of peak detecting function circuits are explained by generating a either saw tooth wave or triangular wave and its peak value is detected by a peak detector. The principles of sampling function circuits are explained by generating a either saw tooth wave or triangular wave and its peak value is sampled by a sample & hold circuit.