This paper presents a consideration of micro-heat exchangers design optimization for the aim of process intensification. Two examples are discussed to illustrate different ways of heat transfer intensification in micro-heat exchangers. To solve the key issue of the link between the micro-scale and the macro-scale, a multi-scale design optimization method using fractal and constructal approaches is introduced. The concept of a novel constructal heat exchanger is also proposed. Copyright (C) 2007 John Wiley & Sons, Ltd.