Three Contributions to the Theory of Sex

If the transference of the erogenous excitability from the clitoris to the vagina has succeeded, the woman has thus changed her leading zone for the future sexual activity; the man on the other hand retains his from childhood. The main determinants for the woman's preference for the neuroses, especially for hysteria, lie in this change of the leading zone as well as in the repression of puberty. These determinants are therefore most intimately connected with the nature of femininity.
-from "The Transformations of Puberty"
He was a pioneer in the study of human sexuality and the impact of sexual desire on human behavior, and this 1905 work is considered among his most important contributions to the field. This is the source of such concepts as penis envy, castration anxiety, and the Oedipus complex that we take for granted as fundamental to understanding human psychology.
In the three essays here-"The Sexual Aberrations," "Infantile Sexuality," and "The Transformations of Puberty"-Freud sets out a theory of human sexuality that continues to influence us today.