Throughout evolution, gamete generation and sex hormone production are the two processes combined in the testis. The local proximity of sex steroid-producing cells and spermatogenic cells allows multiple cellular interactions to occur and thereby facilitates the modulation and/or synchronisation of both testicular functions. This mini review provides an introduction to the vast variety of different testicular cell types, the unique bi-compartmental organization of the testis, the many factors being released in the testis and the different forms of cellular interactions occurring between testicular cells. Selected members of two groups of signal molecules (sex steroids, growth factors) are described in detail and specific examples for the intratesticular actions of signalling factors are presented.