Fifty-five years after discovery of x-rays by Wilhelm Conrad Röntgen in 1895, International Commission on Radiological Protection (ICRP) established a committee on permission internal exposure. Natural and man-made radionuclides can enter human body, cause damage effect and lead to health risk; on the other hand, radionuclides can cure cancers and other non-cancer diseases by irradiating the malignant cells and tissues. The radiation doses delivered to the tissues and organs are inevitable to assess risk or to judge the benefit of application of radiation on humans. In this article, basic methodology for dose assessment of internally deposited radionuclides is reviewed. After brief introduction of interactions and effects of radiation, the biokinetic models developed by ICRP for incorporation of radionuclides in humans are described. Then, the dosimetric formula generalized by ICRP and Committee on Medical Internal Radiation Dose (MIRD) is presented. Treatment of decay products and uncertainty analysis in internal dosimetry are especially addressed. Moreover, applications of internal dosimetry in radiation protection, internal exposure monitoring, radon inhalation dosimetry and nuclear medicine are presented with several calculation examples. At last, the future perspective of internal dosimetry is discussed. In an appendix basic internal dosimetric quantities are provided.