The process or method of measuring the dosage of a therapeutic agent is called dosimetry. Dosage determination, incorporating dose safety and dose efficacy, is a key step in treatment development for all transcranial direct current stimulation (tDCS) applications. tDCS dosimetry can be defined as the calculation of a current dosage intended to induce a specific neurobiological effect with minimum or no side or adverse effects. Most scientific publications about tDCS contain some information about electrode size and the intensity of applied currents, but few give a complete description of dosimetry methodology. At the time of writing, no formal guidelines exist for tDCS application. This chapter provides definitions of dosimetry from both safety and efficacy standpoints. Dosimetry associated with the application of tDCS is based on: 1. mode of delivery (anodal versus cathodal), 2. application duration and treatment frequency (single versus multiple sessions of application), 3. electrode montage, 4. electrode size, 5. current intensity, 6. current density, 7. maximum safe density, and, 8. total charge. A full description of these parameters and how they affect dosimetry is provided. For ease of reference, the current densities and total charges for different intensities, electrode sizes and application times are outlined, as well as a detailed description of the clinical decision-making process for tDCS application. The eight steps in this process relate to: 1. therapeutic goals, 2. characteristics of applied currents, 3. electrode size, 4. montage selection, 5. stimulation site, 6. frequency of tDCS application, 7. application technique, 8. subject/patient specific dosage calculation. We also propose 14 guidelines for safe and effective application of tDCS. The scope and standards of tDCS dosimetry outlined in this chapter are designed to act as a statement of competence in tDCS dosimetry, and provide a useful guide for neuroscientists/clinicians in evaluating the quality, effectiveness and appropriateness of health care services.