Research presented in this dissertation is based on a large in-situ measurement sample of high-frequency electromagnetic field measurements performed on 4252 locations during three years’ time in Republic of Croatia in the vicinity of electromagnetic field sources. On each location six measurements of high-frequency electromagnetic fields from public sources in frequency range from 30 MHz to 3 GHz were carried out, which gives a total of 25512 measuring points. Measurements implemented requirements according to complex measurement method which consists of an advancing set of knowledge and best practice integrated with relevant international standardized methods, laws, ordinances, professional experiences and procedures documented in the framework of the accredited testing laboratory. All locations in the sample are classified in relation to high-frequency electromagnetic fields exposure and based on cumulative exposure quotients calculations with a purpose to improve human and environment protection from possible high-frequency electromagnetic field effects. Classification of all locations is based on risk assessment in respect to the limit of maximum permitted cumulative exposure quotient value set by the ICNIRP (International Commission on Non-Ionizing Radiation Protection). It has been established that predominant risk-based classification groups of exposure to high-frequency electromagnetic fields sources are very small risk and small risk locations while a part of moderate risk locations require special attention. Five locations in a sample belong to significant (unacceptable) risk classification group. On the basis of these results a new monitoring high-frequency electromagnetic field methodology is proposed as a part of social responsibility. On the basis of the research, it has been established that, for the purpose of installing additional electromagnetic field sources, high-frequency electromagnetic field assessment is not rational and it should be complementary with measurements. Therefore, this research presented four algorithms, which give confirmation that exposure assessment should be done on the basis of zero-measurement high-frequency electromagnetic fields before any new source installation, to verify measurement results and to conclude whether assessment results are acceptable. For the purpose of monitoring methodology rationalisation, zero-measurement substitution, given in this research, is used. Zero-measurement substitution is obtained from first-measurement with knowledge and understanding of newly installed high-frequency electromagnetic field sources characteristics.