Knowing the population size of rare diseases or special subpopulations like injection drug users (IDUs) is one of the most important challenges in public health and health surveillance systems but it is difficult to estimate these groups. During the last few years, new methods have been suggested to estimate hidden or hard-to-reach populations, one of which is the network scale-up method (NSUM). The NSUM itself includes measuring the personal network size and estimating the prevalence of hidden and hard-to-count populations. In this paper, we basically discussed the indirect methods of calculating the population size, and the history of NSUM and its concepts, and then addressed the estimation of hidden populations with NSUM and the applicable notes for such populations.