This chapter presents the conceptual basics of measurement including the present and envisaged future International System of Units, the SI. The importance of measurement uncertainty is emphasized and it is outlined that the measurement uncertainty cannot be reduced at will. This is because the Heisenberg uncertainty principle imposes a fundamental quantum limit on the precision of measurements. As an example, electrical measurements and their fundamental quantum limit are briefly discussed. Further limitations of the measurement uncertainty arise from various noise sources in most practical cases. We summarize some of the properties of thermal (Johnson) noise, quantum noise, shot noise, and low frequency (1/f) noise. The SI is described and the present definition of its seven base units and their realization is summarized. In particular, the “thermal” Cs atomic clock is described, which is used to realize the unit of time, the second. Finally, the envisaged new definition of the SI base units is addressed, which is based on fixed numerical values of seven constants of nature.