Congenitally reduced renal mass- as with agenesis of one kidney, unilateral multicystic dysplastic kidney or with premature birth with early arrest of nephrogenesis- as well as acquired loss of a significant part of kidney tissue- as with kidney donation, after surgery for tumor etc- set in motion compensatory processes with main target to meet metabolic body needs. The sensors for reduced renal mass have not yet been identified. The effectors of the compensatory process include a wide range of growth factors- IGF1, TGF-b1, HGF- and signaling molecules-mTOR- which has intricate reciprocal interactions. As nephrogenesis stops at 34-36 weeks of gestation and can't be restarted thereafter, the main result of this compensatory process is increase in glomerular size (glomerulomegaly) and tubular hypertrophy. Renal volume evaluation by ultrasound is a practical noninvasive tool for assessment of compensatory kidney growth. The increased nephron and kidney size induced by the compensatory process have potential detrimental long-term effect through stretch-induced glomerular cell activation of profibrogenic and vasoconstrictor pathways as well as tubular cell nephrotoxicity caused by abnormal activation of reabsorptive mechanisms including GLUT1 and megalin. Deep understanding of these potentially damage process might help in timely implementation of protective strategies.