The term chronic kidney disease (CKD) denotes the persistence of a renal disorder for at least 3 months. CKD is graded as stage 1 when renal glomerular filtration rate (GFR) is normal and from stages 2–5 when the GFR is low and according to the severity of the GFR reduction [1]. Stage 5 CKD equals the classic term "end-stage renal disease" (ESRD) and implies the need of dialysis. The application of this classification to infants needs to take into account that the GFR, expressed in mL/min/1.73 m2, gradually increase during the first months of life and does not reach normal adult values until 1 year of age. In this chapter, CKD will refer to stages with decreased GFR unless the opposite is specifically mentioned.