Using ultrasound to enhance heat transfer has been investigated for decades, yet it continues to draw an increasing number of investigations because of its potential to improve the performance of thermal systems. Here we focus on ultrasound-enhanced natural convection, forced convection, and boiling heat transfer. By comparing the results from a variety of studies we can draw the general conclusion for forced convection and subcooled boiling that keeping the ultrasonic frequency low tends to improve the enhancement compared to higher frequencies, at least in the 10's of kHz range. On the other hand, for natural convection higher frequencies appear to yield greater enhancement. The other general trend observed is that as the mass flow rate in forced convection, the temperature difference in natural convection, or the heat flux in boiling heat transfer increases, any enhancements due to ultrasound tend to decrease and eventually disappear. Determining why this happens, and more importantly clarifying the mechanisms leading to ultrasonic heat transfer enhancement, remain fundamental questions that need to be addressed. Practical ultrasound-enhanced thermal systems will require more investigation of how to efficiently couple the ultrasonic waves to the targeted heat transfer surface or volume, and how to scale up such systems to accommodate real-world thermal applications.