The Torah in Parashat Mesora (Vayikra 15:16) establishes that a man who experiences a seminal discharge is considered Tameh (impure), such that he may not enter the area of the Bet Ha’mikdash or partake of hallowed food (such as sacrifices). The Sefer Ha’hinuch explains that such a discharge occurs only as a result of base, physical desires, and the Torah wanted to convey the message that we are to moderate our involvement in physical pleasures, and avoid excessive preoccupation with such activity. To that end, the Torah assigned a status of impurity to a man who experienced a seminal emission, showing us that we must limit our involvement in physical pleasures, and focus our attention instead on the study of Torah and the service of G-d. A man becomes Tameh after the emission of even a single drop. He himself becomes Tameh, and also anybody who touches the discharge becomes Tameh. A seminal emission brings impurity regardless of whether it occurred intentionally or unintentionally, during sleep. The only exception is when the man did not feel any sensation when the discharge occurred, neither at the beginning nor at the end of the emission; in such a case, the individual does not become impure. If a man had arousing thoughts during the day, and then at night he dreamt about having intercourse, then he must assume that he experienced an emission, even if there is no indication of discharge in the morning. He most likely experienced an emission and the substance was absorbed by his garments or bedding, and so he is considered Tameh. This Halacha applies in all places and at all times, even nowadays. In our time, however, in the absence of the Bet Ha’mikdash, there is no practical application of this Halacha, as in any event we do not go to the area of the Bet Ha’mikdash or partake of sacrifices. Ezra – the religious leader at the time of the beginning of the Second Commonwealth – enacted a law forbidding one who experienced a seminal discharge from praying or studying Torah until he immerses in a Mikveh, or until he has nine Kabim (approximately 12 liters) of water poured on him (like a shower). This provision was enacted in order for people to maintain a high standard of purity, and so that husbands and wives would not excessively engage in marital relations. During Ezra’s time, people observed this law, and men immersed in a Mikveh each time they experienced an emission. Later, however, as the Gemara discusses in Masechet Berachot, the Rabbis realized that this requirement was too difficult for the masses to observe, and so it was annulled. Nevertheless, the Sefer Ha’hinuch writes that it is a “Mida Toba U’meshubahat” – “a good and praiseworthy measure” – to observe Ezra’s enactment and immerse after experiencing a seminal emission. Likewise, the Rambam famously wrote in a letter that although immersing after experiencing an emission is not Halachically required, nevertheless, he made a point of observing Ezra’s enactment, and never missed a single immersion. Thus, although there is certainly no requirement for a man to immerse after experiencing an emission, it is proper for one who can to immerse from time to time in order to maintain a higher level of Kedusha.