Bitches are monoestrous animals in which the reproductive cycle consists of proestrus, oestrus, dioestrus and anoestrus. Puberty occurs between 6 and 23 months of age, on average between 10 and 24 months of age. The reproductive cycle of bitches differs from most mammalian species because they ovulate immature oocytes that cannot be fertilised immediately. Primary oocytes need one more division and maturation before successful fertilisation. Also, the canine reproductive cycle is characterised by an increase in the concentration of progesterone due to the luteinisation of the granulosa cells of the ovary before ovulation. Several methods are used to determine the optimal mating time of bitches, among which the most famous are vaginoscopy, measurement of progesterone and LH serum concentrations, and vaginal cytology. Vaginal cytology is a simple clinical method consisting of sampling, staining and microscopic examination of exfoliated vaginal epithelial cells. The vaginal epithelium thickens as a result of increased concentrations of oestrogen in the plasma. The increased number of cell layers of the vaginal epithelium results in a reduced supply of epithelial cells with blood, their death and eventual shedding into the vaginal fluid. The percentage of certain types of epithelial cells visible in the sample gives us information about the phase of the reproductive cycle of the bitch. Vaginal cytology, along with monitoring the reproductive cycle, plays an important role in the diagnosis of various pathological conditions of the reproductive system.