The crown prince, who is 64, is much less well known to Thais and has not attained his father's widespread popularity. He spends much of his time overseas, especially in Germany.
Profile: Crown Prince Vajiralongkorn
Strict lese-majeste laws protect the most senior members of Thailand's royal family from insult or threat. Public discussion of the succession can be punishable by lengthy jail terms.
Given the pivotal role the king has played in maintaining the balance of power in Thailand's volatile political environment, the succession will be a formidable challenge for the government, says the BBC's Jonathan Head in Bangkok