Outdoor design conditions are important for heating, ventilating and air-conditioning (HVAC) system design and energy estimation for buildings. A research study on the determination of outdoor design conditions for HVAC applications in Hong Kong is presented here. Methods for determining outdoor design conditions are examined and the existing data for Hong Kong are studied. New design data developed from the latest weather database compiled for Hong Kong are provided. The characteristics of the Hong Kong climate are studied from the 33-year long-term statistical distributions of its hourly dry-bulb and wet-bulb temperatures. Significance, properties and proper selection of outdoor design conditions for HVAC design are then discussed. It is hoped that designers can assess critically the outdoor design conditions they have taken for granted for their building design and evaluate suitable data for design weather based on their applications and risk levels.