The Apicomplexa are a large group of protozoa, characterized by the presence of an apical complex at some point in their life-cycle. They are exclusively parasitic, and completely lack flagella or pseudopods except for certain gamete stages. Diseases caused by Apicomplexa include:

• Babesiosis (Babesia)
• Cryptosporidiosis (Cryptosporidium)
• Malaria (Plasmodium)
• Toxoplasmosis (Toxoplasma gondii)

Most members have a complex life-cycle, involving both asexual and sexual reproduction. Typically, a host is infected by ingests cysts, which divide to produce sporozoites that enter its cells. Eventually, the cells burst, releasing merozoites which infect new cells. This may occur several times, until gamonts are produced, forming gametes that fuse to create new cysts. There are many variations on this basic pattern, however, and many Apicomplexa have more than one host.

The apical complex includes vesicles called rhoptries and micronemes, which open at the anterior of the cell. These secrete enzymes that allow the parasite to enter other cells. The tip is surrounded by a band of microtubules, called the polar ring, and among the Conoidasida there is also a funnel of rods called the conoid. Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semi-rigid pellicle.

The presence of alveoli and other traits place the Apicomplexa among a group called the alveolates. Several related flagellates, such as Perkinsus and Colpodella have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the dinoflagellates. They are probably similar to the common ancestor of the two groups.

Another similarity is that apicomplexan cells contain a single plastid, called the apicoplast, surrounded by four membranes. Its function is unknown, but it appears to be necessary for survival. They are generally considered to share a common origin with the chloroplasts of dinoflagellates, although some studies suggest they are ultimately derived from green rather than red algae.

The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group for parasitic protozoans without flagella, pseudopods, or cilia; some authors use the name to refer specifically to this group.