Subfertility affects about 15% of all couples. Assessment of spermatogenesis has a central role in the evaluation of the subfertile couple. Classical markers of spermatogenesis, such as semen analysis, testicular biopsy and endocrine evaluation all have their diagnostic limitations. There is a clear need for accurate additional markers of spermatogenesis. Recently, the serum inhibin B level has emerged as a sensitive endocrine marker of spermatogenesis. This paper summarises the pros and cons of different markers of spermatogenesis, with specific focus on serum inhibin B. The serum inhibin B level has been shown to be associated with classical markers of spermatogenesis, particularly testicular histology, and to be the most accurate endocrine marker of spermatogenesis. A subnormal serum inhibin B level clearly reflects disturbed spermatogenesis. Before puberty, when no spermatogenesis takes place, inhibin B is a marker of testicular integrity. Clinical applications of serum inhibin B in childhood and adulthood are given, and a view on future directions and research is presented. The serum inhibin B level has proven to be valuable in the evaluation of spermatogenesis, and holds a promise for further research.