The transition from early childhood education to primary education is, in many countries, the beginning of compulsory education. This has meant that much of the research on this transition has focused on academic aspects such as literacy or logical-mathematical competence. The lines of research have changed, and in recent years, very diverse aspects have been analyzed in relation to transition. These investigations are usually carried out by analyzing in depth one or several aspects related to transition, but they do not give a general view of the problem. The aim of this study is to bring together all the factors that have been studied on this transition and to achieve an overall view by means of a systematic review. The results have given rise to a model in which more than 300 variables are grouped into 29 groups and, in turn, into an academic and a nonacademic context. Although the largest number of variables studied are found in the academic context, 67% of the articles consider both contexts. The agents involved, as well as educational policies, should consider the complexity of this process and all the factors involved in it, given that an adequate adaptation to the educational system in its early stages will contribute to success in later stages.