Conceptions of childhood have undergone continuous and historical evolution; children can no longer be regarded as small adults. Most contemporary views on the nature of childhood are derived from Aristotelian concepts; they stress its developmental nature and the role of adults in guiding and facilitating children's development. Transformation to adulthood occurs by a process of biological, cognitive, and moral development in which distinct stages can be identified. Children's portrayal in art, literature, and the media has largely mirrored evolving concepts of childhood and increasingly takes account of children's voices.Biologically, children's anatomy, physiology, and pathology go through significant changes. Infants and very young children show an increased vulnerability to environmental and other harms, because of their size, immature anatomy and physiology, and differing pharmacodynamics. Organ development determines the patterns of disease that occur in childhood and affects drug treatments and responses to them.Cognitive development (the child's evolving ability to think and understand) is classically understood as following a series of stages in which genetically determined characteristics interact with events and experiences to influence patterns of cognition. A similar staged pattern is seen in moral development in which the child's capacity to make decisions about what is right or wrong gradually evolves. Contemporary neuroimaging techniques have provided structural and functional evidence to underpin this process. A child's stage of cognitive and moral development is used to determine what legal status they may have and what weight should be placed on their desire to determine their own best interests.Concepts of children as rights holders allow them to have moral agency, irrespective of their cognitive ability or capacity to exercise choices. A staged acquisition of liberty rights (i.e., right to self-determination) seems plausible, as does the universal possession of welfare and protection rights.The interests of children who lack the capacity to exercise liberty rights can be protected by the application of the best interests or welfare principle. Determination of best interests includes consideration of the harms and benefits involved; the likely outcome of various options; the wishes, preferences, beliefs, and values of the child insofar as she/he is able to express them; the wishes, beliefs, and values of the family and other relevant individuals; and which option is least restrictive of future choices for the child.Children are not small adults who can be treated as though they were, and neither are they uniformly vulnerable beings who need protection; rather they are individuals in transition whose growth into adulthood should be supported, encouraged, and facilitated.