Frogs croak, lions roar, peacocks show their tails, but chemical signals for attracting a mate are much more employed. In fact, chemical signaling can be employed also for alarm, communicating food availability, defending territory.
But do humans use the chemical signals?

Pheromones are chemicals which, when emitted by one individual, change another individual's physiology and behavior.
Mosquito and butterfly males have huge antennae for detecting female sex pheromones, while most vertebrates employ the Jacobson organ, placed on the roof of the mouth.

In humans, the Jacobson organ is atrophied and thought to be non-functional.

But researches proved that extracts of sweat from other women induced synchronization in women's menstrual cycles. Other researches showed that women exposed nasally to androstadienone, an androgen-like chemical encountered in male underarm sweat, experienced psychological and physiological responses, even when they did not smell it. But androstadienone was just exposed to the Jacobson organ. It induced a slight anxiety, decreased heart rate, skin conductance and other physiological measurements.

Positron-emission tomography (PET) brain imaging revealed that androstadienone turned on centers in the hypothalamus (a brain nucleus involved in sexual behavior and reproductive hormone control) in straight women and gay men, while in straight men and lesbians it affected just the olfactory centers.

An estrogen-like pheromone, synthesized by women had an opposite effect.

Other researches detected that sniffing androstadienone maintained high the cortisol levels throughout the day, while normally these levels drop over the course of the day.
Many are not convinced that this proves androstadienone as being a (sex) pheromone: measurements of women's sex hormones, like estradiol, progesterone, luteinizing hormone and follicle stimulating hormone (pituitary hormones that control ovarian function) would be more useful. Moreover, most androstadienone studies have tested women before ovulation, when they have higher sex drive and tend to be more drawn into masculine traits in men (possibly including manly pheromones).

High cortisol levels do not necessarily mean distress or negative emotion: cortisol's main function is to mobilize the body, that's why levels rise when we wake up, eat or exercise.

By this point of view, androstadienone would trigger a hormonal response in women to prepare for courtship or sexual activity. But if it is a real sex pheromone, there are more researches to be done to prove it.