In the last 20 years, the use of high voltage direct current (HVDC) power transmission has grown considerably. With the increase in the number of HVDC transmission lines and in the transmission voltages, concern has increased as to possible health effects. For research on these health effects to be carried out, it is necessary to measure the electrical environment ofHVDC transmission lines accurately. The electrical environment ofHVDC transmission lines is characterised by: Electric field strength E [Vim];Ion current density J [A/m²] ;Space charge density p [C/m³ ]. Typical values measured under HVDC transmission lines are: E = lOkV/m, J = 400nA/m² . This thesis describes the design, construction and testing of an electric field mill capable of measuring electric field strength from 0 to 25kV/m (resolution = lOV/m), and ion current density from 0 to 1000 nA/m² (resolution = 20nA/m² ). The electric field readings have an accuracy of ±8%. The ion current density readings have an accuracy of ±12%. The field mill was tested in the laboratory in a simulated HVDC environment, and found to give good performance. For operation under a HVDC line, the field mill design will need some alterations to make it weatherproof. The design considerations described in this thesis should be helpful in the design of electric field mills in general.