The cardinal objective of this research has been to investigate cross- culturally the performance of Egyptian and English children of age’s 5 to 15 on science conceptualization in the light of Piaget's theory of cognitive development. Two integrated empirical studies were administered to a total of 891 subjects from the two countries. The preliminary study assessed the preoperational children's (ages 5 to 11) understanding of the concepts of life and death. The 389 children who participated in this study of animism showed no evidence that animism is a spontaneous tendency of the preoperational child's mental structure universally demonstrated. Chi-square analysis revealed some small significant differences between Egyptians and English, and no significant differences between sexes. Kurskal-Wallis One-Way Anova indicated that there was a correlation^ between ages and correct responses. A sample of 502 subjects (270 Egyptians, 232 English) of ages 11, 13, and 15 participated in the second study of the development of science concepts in the concrete and formal operational children. Nonparametric statistical technique of Mann- Whitney U test was used to compute the data obtained. The results revealed that a small proportion of children attained the late concrete and formal operational stages, that the attainment of the stages was in correlation with age levels, that there was no definite age at which a child attained a specific developmental stage, that there were very slight variations between Egyptian and English children's attainment of the late formal stage, that there was a relationship between gender and the attainment of late formal operation stage, and that the educational system was a factor in the performance of the children on science concepts. These findings tend to support Ausubel's theory rather than Piaget's theory of cognitive development.