Male infertility is often due to defective sperm function rather than production. However, semen analysis identifies only severe motility defects caused, e.g. by primary ciliary dyskinesia (PCD), which can also involve multiple morphological abnormalities of sperm flagella. The causes of these defects remain largely undetermined. Moreover, in many infertile patients, semen analysis yields normal results (normozoospermia), suggesting an unexplained sperm dysfunction, e.g. due to defective key signalling molecules such as ion channels and transporters. Here, similarly, the underlying mechanisms have remained enigmatic. Thus, this research area aims to elucidate the pathomechanisms underlying male infertility due to ciliopathies and unexplained sperm dysfunction,shedding light on the genetic, molecular, and cellular programmes controlling the function of sperm and motile cilia.