Traditionally, education in Saudi Arabia has tended to lay considerable emphasis on the correct recall of memorised information. In the early years of the 21st century, education policy in Saudi Arabia began to consider the introduction of the concept of critical thinking into the curriculum. At the same time, the role and place of the sciences in the curriculum have increasingly been emphasized, the aim being to equip future generations with the skills thought important in taking the country forward. This study is, therefore, set in Saudi Arabia and focuses on the nature and development of critical thinking in the context of the current curriculum in the sciences. After describing the educational scene in Saudi Arabia, the thesis focuses on what is known about thinking in general and critical thinking in particular. The aim here is to move towards the development of a model of critical thinking and some kind of operational description against which test material can be developed. At that stage, it was recognised that, while critical thinking might be conceptualized as a set of cognitive skills, there is a strong attitudinal element. In simple terms, the learner needs to know how to think critically but also be willing to use these skills. There is a very brief review of some key research in the area of attitudes, including the principles of measurement which underpin the way the perceptions and attitudes of the learners are considered in this study. The cognitive nature of critical thinking is then related to two key research contributions of the 20th century: the work of Jean Piaget and David Ausubel. Critical thinking takes place in the working memory and the insights from information processing are discussed, looking at the ways information moves around the brain and the implications for the development of critical thinking are discussed. This study aims to explore how to measure critical thinking and to determine whether critical thinking skills can be developed in science subjects in school pupils. To achieve this aim, a model of critical thinking was first developed representing that thinking critically basically involves asking the questions how, what and why of new sources of information, the information itself and the linking processes involved in understanding. A test of critical thinking was developed based on this model. The data from this test were related to several other educational measures: student perceptions, working memory capacity, understanding science, school marks in science. Interviews with teachers and school inspectors were also conducted to explore their perceptions. The researcher is confident that the model and test make a contribution to the literature, as well as being of benefit to Saudi Arabia and to other countries. Critical thinking was measured with a total of 240 pupils, 120 girls and 120 boys, aged between 13 and 15, in classes 1, 2 and 3, in six Intermediate Schools in Saudi Arabia in the academic year 2009-2010. The questions in the test were designed so that success in the test relied on one or more of the aspects of critical thinking. The outcomes were related to working memory capacity and school science performance while student perceptions were measured. Principal Components analysis using Varimax rotation showed that the test designed to measure critical thinking was not measuring either science knowledge or understanding nor was it a measure of working memory capacity, but the school marks were highly correlated with working memory capacity. It was found that the measured critical thinking grew from year 1 to year 3; possible reasons are suggested. While the validity of the critical thinking test is not certain, it is not simply a measure of knowledge and understanding or of working memory capacity although any critical thinking would take place in the working memory. The survey offered many insights but, in particular, it revealed that most pupils had a negative attitude to science and showed broadly negative perceptions of science. The following experiment aimed to determine whether critical thinking skills could be developed in science subjects in school pupils. A fresh sample for the second experiment consisted of 1,600 pupils, from 12 schools, 800 girls and 800 boys, 400 of each in grade 1 (aged 13) and 400 in grade 3 (aged 15). Of these 400, 200 were in control groups and 200 in experimental groups. The pupils in the experimental groups were taught critical thinking skills using teaching material specifically developed for this research (which took 9 weeks to complete) and with a method proposed for it, while those in the control groups were taught in the normal way. The following were measured: student perceptions, working memory capacity, critical thinking, and understanding. The first two tests were identical to those used in experiment 1 and the critical thinking test was only slightly modified. In addition, their school marks were taken to make a fifth data set. Analysis of the data showed that critical thinking skills grew significantly after use of the new materials, with year 3 showing greater growth. Despite attempts to make the material gender neutral, boys were found to be better at critical thinking skills, although this may simply reflect gender-separated education. Principal components analysis again showed that critical thinking test data is unrelated to measured working memory capacity, measures of recall, and measures of understanding. The student survey was carried out with the purpose of examining pupils’ attitudes towards various aspects of thinking and critical thinking in the context of science teaching and some unexpected gender differences were observed. In the test of critical thinking skills, girls and boys in the experimental groups both performed better than did the control groups but the girls did not appear to be convinced that this is the case.A total of 98 science teachers and science inspectors were interviewed in order to explore their perceptions of critical thinking in science education. A range of themes was explored and there were some differences in their views which, in turn, did not seem to match the views of the students. Implications are discussed. The overall conclusions are that critical thinking can be measured and that it can be enhanced with school learners aged in the range 13 to 15. However, it is vital that educational policies, resources, national assessment and teacher training be adjusted if the development of critical thinking is to make much progress in Saudi Arabia. The limitations and implications of the study are outlined.