• Incumbent Barisan Nasional (BN) MP Wee Ka Siong, deputy president of Malaysian Chinese Association (MCA)
• Mr Liew Chin Tong, Johor chief for Democratic Action Party (DAP), a member of Pakatan Harapan (PH)
• Mr Mardi Marwan of PAS
Dr Wee and Mr Liew play major roles in their parties. A defeat will be read as a big blow to their parties.
• Incumbent MP Muhyiddin Yassin, president of Parti Pribumi Bersatu Malaysia (PPBM). He co-founded the new party after being sacked from Umno amid the 1MDB scandal. He is also contesting the Johor state seat of Gambir for the first time (PH)
• Mr Ismail Mohamed, who took over the Pagoh Umno division as acting chief to replace Tan Sri Muhyiddin. He was moved from a Johor state seat (BN)
• Mr Ahmad Nawfal Mahfodz, a first-time candidate for PAS, is one of the youngest candidates in the elections. His father Mahfodz Mohamed, chief of PAS' ulama (clerics) wing, is challenging Mr Muhyiddin for the Gambir state seat.
PPBM will test Malay voters' loyalty to BN in Pagoh. A win here could herald a wider Malay voter swing towards the opposition.
• Incumbent MP Nur Jazlan Mohamed, Deputy Home Minister from Umno (BN)
• Mr Salahuddin Ayub, deputy president of Parti Amanah Negara (PH)
• Cardiologist Mohd Mazri Yahya of PAS
• Mr Yap Keng Tak , an independent
This is a second-round match for the BN and PH contenders.
Mr Salahuddin is the most senior official in his party taking part in the opposition thrust into Johor.
A win should help Mr Nur Jazlan secure a full ministerial post.
• Tun Dr Mahathir Mohamad, president of PPBM and chairman of PH
• Incumbent MP Nawawi Ahmad, Umno Langkawi division chief (BN)
• PAS Langkawi chief Zubir Ahmad
The contest in Langkawi will be closely watched as former premier Mahathir needs to win to secure his passage to Putrajaya as the opposition alliance's prime minister candidate.
• Incumbent MP and Plantation Industries and Commodities Minister Mah Siew Keong , president of Gerakan (BN)
• Mr Nga Kor Ming, chairman of DAP Perak (PH)
• Dr Ahmad Ramadzan, PAS coordinator for the seat Gerakan, once the party that ruled Penang, is today left with few seats, including Teluk Intan.
• Incumbent and Transport Minister Liow Tiong Lai, president of MCA (BN)
• Mr Wong Tack, environmental activist from DAP (PH)
• PAS Supporters Club chairman N. Balasubramaniam
This is a rematch between Datuk Seri Liow and Mr Wong. In 2013, the former won by the narrowest of margins, just 379 votes.
• Mr Nasrudin Hassan, PAS information chief
• Mr Saifuddin Abdullah, PKR, also chief secretary of PH secretariat
• Dr Johan Mat Sah of BN This seat will see a hot rematch between old rivals Mr Nasrudin and Datuk Saifuddin that could split the opposition vote, benefitting BN. Mr Nasrudin beat Mr Saifuddin by just 1,070 votes in 2013 in the federal Parliament seat of Temerloh in the same state.