Azoospermia means absent of sperms in the semen. Causes of Azoospermia are:

1. Problems in the Pituitary gland in the brain- due to tumour, head injury, birth defects, some medicines or surgery 

2. Inability of the testicles to produce enough sperms- due to mumps orchitis, varicocele (sweling of veins of the testicles), undescended testes (testes not present in the scrotum at birth), infections (tuberculosis), injury, previous chemotherapy or cancer treatment, excessive smoking and some medicines, chromosomal problems etc 

3. Blockage in the sperm conducting ducts- due to hernia surgery, infection, cystic fibrosis, blcok since birth etc.