Employee engagement is an important issue in management theory and practice. However, there are still major differences in the concept, theory, influencing factors and outcomes of employee engagement, and there is still no authoritative standard. This paper attempts to review and summarize previous research results on employee engagement. Two kinds of definitions of employee engagement are identified: employee engagement as a multi-faceted construct (cognition, emotions and behaviors) and as a unitary construct (a positive state of mind, a dedicated willingness, the opposite of burnout). Three theoretical frameworks are used to explain the varying degrees of employee engagement: Needs-Satisfaction framework, Job Demands-Resources model and Social Exchange Theory. The influencing factors of employee engagement are divided into three categories: organizational factors (management style, job rewards, etc.), job factors (work environment, task characteristics, etc.) and individual factors (physical energies, self-consciousness, etc.). Employee engagement is found to have a positive relationship with individual performance (organizational commitment, positive behavior, etc.) and organizational performance (customer satisfaction, financial return, etc.) The research findings show that there are three shortcomings in previous studies: lack of research on demographic variables, personality differences and cross-cultural differences in employee engagement, lack of research on the mediating or moderating role of employee engagement, and lack of intervention mechanism for employee engagement.