Design and architectural characteristics of paediatric or neonatal intensive care units (PICU/NICU) are important but often neglected. They may have a significant impact on the performances of care delivered in these units. There are some recommendations on these issues available in the literature although most of our knowledge is related to the ‘physical’ experience accumulated by colleagues working in units with different settings and characteristics. In this chapter we will describe the main basic characteristics of the modern PICU/NICU design and try to understand how these designs may answer the clinical needs. We will cover both advantages and disadvantages of each of them. We will particularly stress the need to understand the critical threshold values of light, noise, power and humidity which must be accurately controlled. Some suggestions about choosing between the different designs will be given with reference to both PICUs and NICUs.