To help sports organisations and their practitioners understand the influences of growth and maturation on youth athletes, this chapter has several objectives. First, the potential inter-individual variability in growth and maturational development across and within chronological ages is explained. To assist, a conceptual model demonstrating growth and maturation-associated changes over time is provided. Second, relationships between physical maturation and facets of athletic performance are highlighted. How these relationships lead to systematic biases in athlete evaluation and selection are described. Third, the implication to better consider growth and maturational differences, and measurement issues are discussed to better inform physical assessment of athletes.