Entrainment phenomena in capillary-driven heat pipes were studied and both analytical and experimental approaches were utilized to identify and better understand the parameters that govern the entrainment of liquid in operating heat pipes. A heat pipe experiment was conducted using a high power heat pipe, designed to verify the existence of the various modes of entrainment and measure the corresponding entrainment limits. In addition, entrainment limit data were theoretically verified using a computer model designed to predict the maximum performance for the given operating conditions. Entrainment was detected by various methods and classified into representative types according to the relative position of the liquid interface to the wick structure and the operating conditions. Results of the experiment were compared with those obtained from previous investigations presented by Cotter, Tien and Chung and Prenger and Kemme. From the comparison of the critical Weber number or-vapor velocity, the effectiveness of previous aerodynamic simulations was also examined.