Small field dosimetry has evolved actively since 2005 with the development of advanced treatment modalities. The conceptual foundation of dosimetry is relatively clear now, with a vast amount of literature on the subject. This chapter explores historic development, misinformation, and poor understanding, and attempts to provide a clear picture of the field of dosimetry, incorporating lateral disequilibrium, source size occlusion, and selection of detectors. Even though IAEA-TRS 483 has provided correction factor data, this chapter debates the concept of the definition of dosimetric field size (Sclin) and provides accurate data. The output factors are elaborated in the light of source size in conjunction with Monte Carlo data. Recent developments and advances in small field dosimetry indicate that the Monte Carlo simulations not only provide an accurate, comprehensive beam dataset that can be verified by experimental measurements but also shed light on the sensitivity of experimental uncertainties, especially for extremely small fields. It also narrows down the choice of detectors in small field dosimetry among the vast number of detectors.