Leeches are incapable to breed asexual. Their regeneration ability is also very poor. When a leech is divided into two parts, the anterior part can dorm a new anus, but the posterior part cannot form a new head (Davies, 1991; Çağlar, 1973; Savyer,1986). All the leeches are hermaphrodites, and have one male and one female gonopore in their reproduction systems. Male and female organs open to the outside of the body in the median part of the ventral part. The male genital pore is always located at the anterior of the female pore. Male gonads are in the form of round sacs. The number of these can be at least 4 couples, or 17 couples at the most. Testes are aligned at sides, one couple in each segment of the mid part of the body. One spermatic canal extends at the outer sides of rows of testes. Each testis opens with separate channels to the sperm canal, which belongs to its side. Both large seminal ducts extend forward. This sperm sacks, which is considered as a seed-bag, leads to a common atrium after expansion. The common end of the spermatic canals in Hirudinidae contains a penis (Figure 5). Atrium is surrounded by secretory cells, opening to a very short and complex lumen.

The proximal secretory gland has been named as prostate. Spermatophores have not been developed enough. Spermatogenesis starts in the lumens of testes initially in leehes. Rhynchobdellida and Pharyngobdellida have no copulation organs (Barnes, 1974; Davies 1991; Kaestner, 1967; Sawyer 1986). Number of ovaries is always limited with one couple, and they are located between the anterior couple of testes and the male atrium. Ovary is an assemblage of the germinal tissue, and sometimes it can contain a few cocoons (sac of ova) inside. (Barnes, 1974; Davies 1991; Kaestner, 1967; Sawyer 1986). The short oviduct lays in the anterior direction and unites with the organ coming from the other side of the vagina. The female gonophore opens at the ventral side of the 11th segment. Ova are released to the ovaries as immature gametes (oocytes) and complete their maturity within the cocoon fluid. (Barnes, 1974; Davies 1991; Kaestner, 1967; Sağlam and Sarıeyyüpoğlu 1998; Sawyer 1986).

Insemination of some kind of leeches is carried out after the transfer of sperms to the vagina of the other leech through the penis (Gnathobdellida). In others, a copulation tool containing chitin is formed at the tip of the seminal duct (spermatophore) that are stabbed into somewhere on skin of the partner (mostly at the dorsum) (Rhynchobdellida, Pharyngobdellida). Spermatophores have dissolving effect on the skin. Sperms are transferred into the coelomic space in a little while and from there, to the ovaries. In general, insemination carried out with penis is one-sided, while the same carried out with spermatophores is mutual. After a period following insemination, ova are transferred to a sac filled with a nutrient fluid containing albumin secreted by clitellum glands (Figure 8). Clitellum becomes clearly visible within this period. (Barnes, 1974; Davies 1991; Kaestner, 1967; Sağlam and Sarıeyyüpoğlu, 1998; Sawyer 1986). The sac of ova carry the productive eggs after being released from the female gonophore (Figure 6). One cocoon carries only one ovum in Piscicolidae; however, this number varies in other leech species. Nephelopsis obscura leaves 1 to 4 cocoons in an egg-laying period, and each cocoon carries 1 to 8 eggs (Sağlam and Sarıeyyüpoğlu, 1998). Hirudo medicinalis however, produces 1 to 8 cocoons in a period, and each cocoon can contain 33 embryos (Savyer, 1986). Sacs are mostly left on objects in water; some parasitic forms leave them within the body of the host. There are also other leech species, like the Hirudo medisinalis, which leave the water and are burry in moist soil to release the cocoons. Among the Hirudinea there are those who care for the young ones.

These forms either carry the eggs with them or protect the young ones while staying where they have left their eggs. Leeches mature without metamorphosis ((Barnes, 1974; Çağlar, 1973; Kaestner, 1967; Sawyer 1986). Most of the leaches survive for one year. Leaches hatching in spring mature within the following year. Their lifecycle depends on their feeding habits and habitat. Hirudo medisinalis live much longer than other species. (Barnes, 1974 ).