The common use of the term personality is in designating social effectiveness, as when we say ‘she has a lot of personality’. Another viewpoint is to regard personality as an individual’s most striking characteristic. However, when psychologists discuss personality they are focusing on individual differences, those attributes that distinguish one person from another. These differences can be noted soon after birth with activity levels, attention span and reaction to changes in the environment being the major ones reliably observed. The chapters on development, child-rearing practices and self-concept reveal that most aspects of personality are learnt through interpersonal relationship within the family and immediate environment. But the differences at birth noted above and the concordance of such features as schizophrenia to personality as well. This chapter identical twins suggests a genetic determinant to personality as well. This chapter will concern itself with some of the major theories that have been produced to explain individual differences in personality which is best defined as ‘those characteristic patterns of behaviour and modes of thinking that determine a person’s adjustment to the environment’.