Muhammad (c.570 - 632) is regarded by Muslims as the greatest and last of a succession of prophets sent by Allah (God). Muhammad's teachings gave rise to the religion of Islam. The Qur'an was allegedly revealed to Muhammad through the angel Gabriel who was sent by God. Muhammad received this revelation over the course of 23 years. While Muslims do not consider Muhammad to be divine, he is held in extreme reverence.
Muhammad was a merchant, religious leader and military general of converts. He claimed to have received a revelation from Allah that there was one monotheistic god. Muhammad spent many years in Mecca attempting to gain converts to his new religion with limited success. Fleeing from persecution in 622, he travelled to Medina with his followers. While based in Medina, he united the surrounding tribes and conducted a war against the Meccan tribes. In 629, Muhammad finally lead an army that captured Mecca and destroyed the pre-existing pagan polytheistic temples and idols. This was followed by various military expeditions against nearby pagan tribes that refused to convert to Islam.  Before his death by illness in 630, most of the Arabian peninsula had converted to Islam.
Muhammad is said to have had thirteen wives, mainly for political or humanitarian reasons. He had several children and grandchildren. He was a slave owner, which was a common practice at the time.
Honorific "Peace be upon him"
Muslims usually append "Peace be upon him", PBUH or the Arabic equivalent after Muhammad's name or any other prophet. This is based on the saying attributed to him by a hadith:
- "The miser is the one in whose presence I am mentioned, then he does not send the Salah [prayer] upon me."
This practice may be ignored by non-believers.
Depiction of Muhammad
Many modern Muslims consider depiction of Muhammad as potentially idolatrous and almost always blasphemous. While there is no explicit ban on depiction in the Qur'an, depicting any prophets is forbidden in a few hadiths. While some sections of Islam are more tolerant of depictions, there are many who believe that no depictions, even by non-believers, are allowed. This is obviously contrary to freedom of expression that is practiced in many non-theocratic countries.
Several writers and artists have been threatened based on their Islamic themed art, such as the still current fatwā and failed assassination attempts targeting Salman Rushdie which forced him into hiding for many years. Another notorious case was the Jyllands-Posten cartoons, which lead to protests and unrest in several middle eastern countries and lead to several deaths. In 2015, gunmen attacked the offices of the French magazine Charlie Hebdo and killed 12 people - mostly journalists. The gunmen were likely motivated by the magazine's depiction of Muhammad in satirical cartoons.  Violence is a gross overreaction to the work of non-violent artists and journalists, as well as being incompatible with freedom of speech and an open society.
Some governments have argued that certain aspects of religion, including criticism or depiction of Muhammad, should be banned on the grounds of religious defamation. This is contrary to the principle of freedom of speech.
Muhammad performed many questionable activities in his life. Criticism of them is sometimes labelled as presentism  and generally avoided by historians. However, if Muhammad claimed to have a timeless objective moral code, it is appropriate to discuss his actions in a contemporary moral context.
Muhammad treated his enemies without mercy, including ordering the beheading of hundreds of surrendered prisoners, assassinations, ordering torture, selling women and children of defeated tribes into slavery and exiling whole tribes. Apologists dispute some of these claims and point out that many of these actions were typical for the time. He conducted offensive wars against enemies, including those who refused to convert to Islam. 
Ownership of slaves
Muhammad is also criticised for owning, enslaving, selling and buying slaves. He also legalised marriage of one's slaves and to have sexual relations to them, regardless of consent Surah 4:24 . Again, this was common practice at the time. Muhammad also encouraged slaves to be freed. He also discouraged the forced prostitution of slaves and only allowed enslavement of people in specific circumstances.
Polygynous marriages, as practiced by Muhammad, are condemned in most industrialised countries. Muhammad's wife, Aisha, was very young when betrothed (6-7 years old) and when the marriage was consummated (9-10 years old). The Qur'an, written by/revealed to Muhammad stipulated that a man may have up to four wives Surah 4:3 . Muhammad's fifth wife, Zaynab, was the ex-wife of Muhammad's adopted son Zayd Surah 33:37 and was contrary to the practices at the time.
When founding a religion, one might aim for a higher calibre teachings than found the in Qur'an.
Defence of Muhammad
As Muhammad is considered a role model by most Muslims, his actions need to be justified particularly when they are contrary to current social norms.
Muhammad's actions need to be considered in context
One argument is to point out that Muhammad's actions were typical or even morally superior to the culture and context that surrounded him.
- "Such an early marriage was normal in all Semitic societies - such as the ones that Abraham(P), Moses(P), Jesus(P) and Muhammad(P) grew up in! "
However, this argument fails because Muhammad supposedly had access to an objectively perfect moral system. Objectively moral systems are timeless, applicable across cultures and apply the same today as when it was first proposed. Assuming Islam is true, the context is irrelevant when considering if an action is moral.
- "If Muhammad was just a fallible human being, who could just as easily be wrong as anyone else, who could just as easily accept some horrible atrocities going on in their name, as human beings have done throughout history, then sure, I'll buy the context argument. But you are not saying Muhammad is just a man, you are saying that he is the chosen one. [...] He is special, he is better than the rest of us. "
The context argument is based on moral relativism, which most theists have already rejected.
Muhammad's actions were commanded by Allah
This argument, based on divine command theory, says that whatever God commands is morally good. Muhammad's actions were supposedly sanctioned by Allah. This is almost a non-defence because it raises the issue of how a "moral" God could command atrocities and actions that today are considered unacceptable. Such a God is unworthy of worship.
Christians cannot criticise without being hypocritical
- "it is grossly hypocritical for Christians to criticise the Prophet's(P) marriage to cAishah at such a young age "
Early marriage was a benefit to Aishah and all Muslims
- "cAishah's marriage to the Prophet Muhammad(P) at an early age allowed her to be an eyewitness to the personal details of his life and carry them on to the succeeding generations. "
This also allowed Aishah to be a role model for later Muslims. However, this argument is not particularly effective since Muhammad could have taken an older wife to fulfill the same role.
Records of Muhammad's actions have been misinterpreted
Some apologists dispute that specific controversial actions even occurred at all, because of unreliable sources or misinterpretation.
- "there are discrepancies/contradictions regarding what her [Aisha's] age was at the time of marriage [to Muhammad], ranging from 9 to 20 years old. "
Critics are biased against Islam
- "To have an atheist, agnostic - or anyone else who does not believe in a Divinely revealed basis for morality - criticize something that is "politically incorrect" by today's moral standards comes as no surprise. Such people will always find something to criticize, since they simply have a bone to pick with "religion" in general. "
This is an ad hominem argument.
Gallery of depictions, including those published by Charlie Hebdo and Jyllands-Posten Muhammad