The Housemates are the official residents of the Big Brother House.
In the show, each contestant is called a "housemate". These housemates are a group of strangers chosen to live inside Big Brother's house for a certain number of days. The housemates must survive weekly evictions to reach the finale and win the grand prize.
Depending on the season's format, theme, and twists, the housemates may receive advantages, power, and punishments from Big Brother. Further, some Big Brother editions may feature various types of housemates.
Types of Housemates
Throughout the Big Brother franchise there are three general types of housemates that appear on their respective editions.
In some seasons, these types of housemates may compete against each other in one season as part of that season's theme or twist, known as the All In twist, also called as Civilian vs. Celebrity on versions without Teen Editions.
Civilian Adult Housemates
The Civilians are the main contestants of Big Brother, and they appear on regular, main editions of every franchise.
Civilian housemates or houseguests are adult people whose ages range from 18 to 75 years old. The civilians are everyday people from different walks of life and often have diverse life stories, religions, gender, personalities, ethnicities, and relationships. 
Civilian Housemates appear on every international Big Brother franchise. They are considered the main contestants of most international versions, except for India's Bigg Boss which typically cast celebrities in its main seasons.
Celebrity Housemates are famous personalities who volunteer to live in Big Brother House. These celebrities usually come from various fields with different levels of fame, such as television, film, theater, music, fashion, sports, politics, social media, and more.
They mainly appear in the celebrity edition of Big Brother. These celebrity editions are commonly known as a separate edition or spin-off, such as Celebrity Big Brother or Big Brother VIP.
In earlier versions of the show, Celebrity Housemates initially played for their chosen charities. Nowadays, the more recent celebrity contestants are often invited to join the show and get paid for appearing in a season.
Most international versions cast Celebrities on their celebrity editions, with Big Brother UK being notable for casting internationally famous and controversial personalities in its Celebrity Big Brother series. 
Teen Housemates are everyday adolescents whose ages range from 13 to 19 years old. These teenagers are similar to civilian housemates in all aspects except their age. Like civilians, the teens come from diverse backgrounds and have different personalities, gender, ethnicity, and religion. Some teens are high school students, while some are in college. Out-of-school teenagers have also entered Big Brother's house.
Teen housemates typically appear in Teen Editions of the franchise. Throughout their stay inside the house, Big Brother assumes as their guardian due to their age. Because of it, Big Brother often becomes stricter to them compared to adult housemates. Further, there are additional age-appropriate house rules that are imposed only among teen housemates. Teen Editions typically air every summer when teens are not in school.
Only two versions have featured all-teen casts in their seasons, Big Brother UK and Pinoy Big Brother. However, the Philippine version is notable for successfully casting teen housemates throughout its series, many of whom became the country's top teen stars after joining the show. 
Notable Housemate Twists
Throughout the Big Brother franchise, several contestant-related twists have appeared in various seasons and versions.
- Estranged Siblings: This twist was notable for reuniting two estranged siblings inside the Big Brother House. The twist first appeared as the main element of the Project DNA twist of Big Brother 5 (US). Years later, the twist appeared in Pinoy Big Brother: Otso.
- Twin Twist: Twins are commonly involved in twists that appeared on various Big Brother franchises. In most seasons where the twist appeared, the twins are usually given a secret task to switch identities and play as one housemate without the rest of the house noticing their swap.
- Pair Housemates: In some seasons, one or more housemates play as one pair. Depending on the season's theme, the pair could be related, friends, lovers, ex-lovers, etc. This twist was used as the main theme of Big Brother 9 (US) and Big Brother 13.
- International Housemates: Foreign-born housemates enter the house and play against local-born housemates. This twist was a major twist in Pinoy Big Brother: Teen Clash 2010 and Big Brother Canada 4.
- Returning Players: Housemates from previous seasons come back to the house for another chance to win.
- All Stars: Fan-favorites, Past Winners, and remarkable players return to the house and compete against each other.
- Big Brother UK is the first Big Brother franchise to cast famous celebrities, and the first version to produce Celebrity Big Brother.
- Pinoy Big Brother is the only franchise to produce a Teen Edition after Big Brother UK.
- Pinoy Big Brother: All In is the first ever season to feature all three types of Housemates living together inside one house.
- The All In twist was adopted by other versions while excluding the teen element of it, and was alternatively called as Civilian vs. Celebrity.
- For the first time in Big Brother history, Gran Hermano 18 (Spain) introduced 100 Civilians inside the house on launch night where 83 of them would be mass evicted the following day and only the chosen 20 housemates would remain.
- Bart in 't Veld is the first civilian housemate to win Big Brother in its history. His historic win took place in the Netherlands back in 1999.
- Jack Dee of Celebrity Big Brother 1 (UK) is the first Celebrity to win Big Brother.
- Paul Brennan of Teen Big Brother: The Experiment is the first teen housemate to win Big Brother.
- Kim Chiu is the first teen housemate to win an international adaptation of Teen Big Brother. Kim also holds the record for being the youngest housemate to ever win Big Brother. She won Pinoy Big Brother: Teen Edition 1 at age 16.
- Werner Hansch is the oldest housemate to win Big Brother. At age 82, he won Promi Big Brother 8.
- Sascha Sirtl of Big Brother Germany holds the record for the most days spent inside the Big Brother house, with a total of 527 days.
- Jodi Rollins of Big Brother US holds the record for the least time spent inside the Big Brother house, with only 6 hours before getting evicted.
- Bailey May of Pinoy Big Brother: 737 is the youngest housemate to enter the Big Brother house. He entered the season's Teen Batch at age 12 (he later turned 13 inside the house).
- Lionel Blair of Celebrity Big Brother 13 (UK) is the currently the oldest housemate to join Big Brother. He joined the season at age 85.