Madeleine McCann was just three years old when she disappeared in 2007.

The toddler was holidaying with her family in Portugal when she went missing, while her parents were dining at a nearby restaurant.

There has never been a credible sighting of her since.

At around 10.00pm on May 3, 2007, British doctor and mum-of-three Kate McCann left a local tapas bar and walked the 83 metres back to her holiday apartment to check on her sleeping children.

The Disappearance of Madeleine McCann trailer. Post continues after video.

Kate, her husband Gerry, their three-year-old daughter Madeleine, and their two-year-old twins Sean and Amelia, had travelled to the area from Leicester, England, for a family holiday.

When Kate arrived at apartment 5A she immediately felt that something wasn’t right.

The door to the children’s room was further open than she had left it. When she looked in the room she saw Madeleine’s bed was empty. A gust of wind then blew into the room, revealing the open window and window shutters on the wall opposite Maddie’s bed.

Kate ran back to the tapas bar to where Gerry was dining with the seven friends they had travelled with, the group that would later become known as the Tapas 7.

She screamed: “They’ve taken her”.

The group quickly began searching the apartment and the surrounding area but there was no sign of Maddie. The McCanns then reported the toddler as missing.

In the following days, Portuguese, British and international press flocked to the seaside village. The story of the disappearance of a blonde-haired toddler in what was considered to be a safe family holiday hot spot was splashed across newspapers around the globe. It became one of the most sensationalised stories of the 21st century.

For the past 11 years Portuguese police, Scotland Yard, and Maddie’s parents have worked tirelessly trying to figure out what happened to the young girl. But so far, they have no answers.

Here are all the theories about what happened to Madeleine McCann:

The sex trafficking theory

One of the most prominent theories is that Madeleine was snatched by a sex trafficking network.

The Netflix documentary, The Disappearance of Madeleine McCann, looked at this theory in depth. It’s believed that Madeleine, a young white girl from a wealthy family, would have been a valuable target for child sex traffickers.

This theory would explain why Madeleine’s body has never been found and why there have been various unconfirmed sightings of her around the world.

The Portuguese predator theory

Some investigators believe Maddie could have been the target of a lone sexual predator who was operating in the area at the time.

As covered in the documentary, the police looked into a man who was wanted in connection to the sexual assaults of five young girls in Portuguese holiday resorts.

Investigators described the man as a "lone wolf" who attacked the girls while they were sleeping in their beds at night. Two of these attacks actually occurred in Praia Da Luz.

This particular lone wolf was one of many operating in the area at the time of Maddie's disappearance.

"One startling element is the sheer number of sexual predators in the area at the time," author Anthony Summers explains in the documentary.

Death by accident theory

Another theory explored in the documentary is that the McCanns could have accidentally killed Madeleine.

As the documentary lays out, the Portuguese police investigated the theory the couple might have killed the toddler with an overdose of Calpol, a sleeping aid.

The Disappearance of Madeleine McCann also looks at how sniffer dogs used in the case allegedly detected the scent of blood, and a human body, in the McCann's apartment and in their hire car.

People who believe this theory also point to the fact Kate McCann refused to answer 48 questions during her interview with the Portuguese police.

The inside job theory

After watching the documentary, many people have suggested that all three of the McCann children could have been drugged, while they were playing at the kids' club earlier in the day.

Apparently Madeleine was very sleepy when she returned to the holiday apartment and went straight to bed. Her younger siblings, Amelia and Sean, also didn't wake throughout the night as people came and went from their bedroom.

The theory is that someone drugged the children to make it easier for them to snatch Madeleine undetected while the parents were at dinner.

The childless couple theory

Another theory is that a childless couple stole Madeleine to raise as their own.

People who believe this theory think the couple either broke into the apartment themselves to abduct Madeleine, or arranged for someone else to take her and bring her to them.

The documentary speaks to a man who claims a woman approached him in Barcelona the night Maddie disappeared. The man says the woman spoke English, with possibly an Australian accent, and asked him, "Do you have the girl? Do you have my new daughter for me?".

When it became obvious to her that he had no idea what she was talking about, she quickly walked away.

The tourism theory

Another theory is that something nefarious happened to Madeleine, but the Portuguese police tried to pin it on the McCanns because they didn't want tourism to suffer.

This theory comes from Brian Kennedy, the wealthy Scottish businessman who funded some of the search for Madeleine. Kennedy talks about this theory in The Disappearance of Madeleine McCann.

The wandered off theory

Possibly the saddest of all theories is that little Madeleine McCann woke in the middle of the night, wandered off in search of her parents, and drowned in the nearby ocean or was hit by a car.

As the documentary explains the sliding door to the apartment was unlocked and the apartment was close to the road and within walking distance to the beach.

You can watch all eight episodes of The Disappearance of Madeleine McCann on Netflix now. 

For more stories on Madeleine McCann, you can read more here:

• Inside the sex trafficking theory linked to Madeleine McCann's disappearance.
• What the Madeleine McCann documentary didn't include.
• What we know about the little known twin siblings of Madeleine McCann.