In concurrent flow of two phase mixtures there exists a velocity difference between the vapor and liquid phases. This difference in velocity is known as the slip velocity. The prediction of slip is the subject of Part I. In boiling systems there is some heat transfer rate at which nucleste boiling becomes unstable. At this point the separate bubbles coalesce forming an insulating vapor film on the heat transfer surface resulting in the destruction, or burnout, of the heater. The prediction of the conditions causing burnout is the subject of Part II.