The first part of this book is dedicated to the problem of measurements of the basic electric quantities in electric utilities, both from the aspect of accuracy of this type of measurements and the possibilities of simple and practical realization. The second part presents a reconstruction of trigonometric polynomials, a specific class of band-limited signals, from a number of integrated values of input signals. The third part deals with the problem of estimating the value of the active power of the ac signal in the presence of subharmonics and interharmonics. The analysis makes use of the most general model of the voltage and current signal, i.e. the most complex spectral content that can be expected to appear in practice.