The relationship between lifestyle and semen quality is of popular interest to patients and the general public. However, new findings are often accepted uncritically without reference to the biological complexities of the male reproductive system. Many factors influence semen quality, including: (i) the inherent sperm production capacity of the testicles; (ii) the period of time since the last ejaculation; (iii) general health; (iv) the duration and nature of pre-ejaculatory sexual stimulation; and (v) the structural and functional integrity of the male reproductive tract, ejaculatory ducts and accessory glands. Each of these factors will have its own bearing on ejaculate quality and any proposed lifestyle factors need to be interpreted with them in mind. This lecture will discuss two important checkpoints where lifestyle factors may have an important effect on adult semen quality. First is the pre-natal exposure of the male foetus to risk during its time in utero through maternal behaviors such as diet, the use of cosmetics and smoking that may have a detrimental effect on testicular development that in turn impact on adult testicular function. Second is the exposure of adult men to factors that affect the function of the post-pubertal testicle and risks are thought to include the effect of temperature (e.g. hot baths, tight underwear), poor diet and a number of chemical exposures either in the workplace or at home. Whilst lifestyle is an important risk factor in male infertility, its potential effects should be taken in context with other biological variables and known medical conditions.