This study seeks to assess the transition probabilities between smoking and alcohol consumption trajectories for ages 13-17 using data from the Korea Youth Panel Survey (KYPS). Four smoking trajectories were identified - noninitiator, late-onsetter, experimenter, and escalator. Similarly, four alcohol consumption trajectories were identified - noninitiator, late-onsetter, experimenter, and chronic user. Those in the chronic group of alcohol consumption were most likely to be smokers. Those who fell into a particular group for use of one substance were most likely to fall into the corresponding group for use of the other substance. Implications for smoking and alcohol prevention are discussed.