Every child while growing up goes through four stages of cognitive development which were defined by the Swiss psychologist Jean Piaget. Those stages are sensorimotorical, preoperational, concrete operational and formal operational phase. Each phase is characterized by different understandings of the concept of the disease, the ways of coping with the disease and the adjustment to the hospitalization. The understanding of the concept of disease in small children was explained in detail by researchers Bibace and Walsh (1980.) who divided each Piaget phase into two more phases whose names describe the leading characteristic of each phase. According to them, there are: phenomenism, contagion, contamination, internalisation, the physiological and psychophysiological phase. Furthermore, Dutch and Swedish scientists created a TEC model in which they are focused on children’s ideas about the causes of the disease and the model itself is correlated with the Bibace and Walsh theory (1980.). A comparison of results in various researchers by different authors on the understanding of the concept of disease in small children leads to the conclusion that children in the preoperative phase interpret the disease with the help of magic and superstition. Children in a specific operative phase know a number of rules for maintaining health and avoiding disease, but do not understand how they work. Representatives of the formal operative phase are closest to the adult level of understanding the concept of disease. The last phase is distinguished from the others by the fact that children separate the physical from the mental health and use their feelings when describing the disease. Pseudo-maturity and regradation to earlier behavior are common changes that occur among ill children, so it is important to recognize the signs of such behavior in order to treat the child properly. Most children go through three phases of adjustment to the hospital environment upon entering the hospital: the protest phase, the despair phase and the apparent adjustment phase. Proper preparation for hospitalization, enabled play and quality communication significantly reduce the childs anxiety and have a positive effect the outcome of treatment. The key to successful treatments is in the medical staff who have quality communication skills and who use a holistic approach. Extensive knowledge, the ability to recognise the changes in an ill childs behavior, educating both the parent and the child, providing comfort and lots more are just some of the skills a pediatric nurse posseses which contributes to increasing the childs quality of life and his environment.