Part I: Definitions and Epidemiology -- Definitions and Relevance of Unexplained Infertility in Reproductive Medicine -- Definitions and Epidemiology of Unexplained Male Infertility -- Controversies Surrounding the 2010 World Health Organization Cutoff Values for Human Semen Characteristics and Its Impact on Unexplained Infertility -- Definitions and Epidemiology of Unexplained Female Infertility -- Part II: Pathophysiology: Male -- Sperm Biology from Production to Ejaculation -- Idiopathic Infertility: Survival and Function of Sperm in the Female Reproductive Tract -- Potential Male Etiologies of Unexplained Infertility -- Genetic Basis of Unexplained Male Infertility -- Role of the Immune System in Unexplained Male Infertility -- Oxidative Stress in Unexplained Male Infertility -- Role and Significance of Sperm Function in Men with Unexplained Infertility -- Role of Environmental Factors and Gonadotoxin Exposure in Unexplained Male Infertility -- Part III: Pathophysiology: Female -- Potential Female Etiologies of Unexplained Infertility -- Oxidative Stress in Unexplained Female Infertility -- Role of Environmental Factors and Gonadotoxin Exposure in Unexplained Female Infertility -- Cervical Hostility and Vaginal pH in Females with Unexplained Infertility -- Role of Endometrial Receptivity in Unexplained Infertility -- Fallopian Tube Dysfunction in Unexplained Infertility -- Subtle Endometriosis and Unexplained Infertility -- Part IV: Evaluation -- Evaluation of Women with Unexplained Infertility -- Evaluation of Men with Unexplained Infertility -- Part V: Expectant, Medical and Surgical Treatment -- The Role of Expectant Management for Couples with Unexplained Infertility -- Treatment and Prognosis of Immunological Factors in Unexplained Male Infertility -- Medication Strategies for the Male with Unexplained Infertility -- Clomiphene Citrate for Women with Unexplained Infertility -- Aromatase Inhibitors in the Treatment of Unexplained Female Infertility -- Gonadotropins for Women withUnexplained Infertility -- Part VI: Assisted Reproductive Techniques and Future Perspectives -- Gonadotropin in Assisted Reproduction: An Evolution Perspective -- Intrauterine Insemination in Unexplained Female Infertility -- Role of In Vitro Fertilization in Unexplained Female Infertility -- Role of Assisted Reproduction Techniques in the Management of Unexplained Male Infertility -- Future Perspectives in the Diagnosis and Management of Unexplained Male Infertility.