Islam pays particular importance to leadership (Imamate). During the Prophet's glorious life, he himself was the leader and chief of the Muslims, in addition to his being a Prophet and a preacher of Allah, the Most High.
Allah, the Almighty says:
"Only Allah, your Guardian and His Messenger and those who have faith, those who keep up prayer and pay poor-rates (zakat) while they bow."
Holy Qur'an (5:55)
Many scholars and other exegists and compilers of the Prophet's traditions, among them Zamakhshari and Wahidi, have stated that this verse was revealed in favour of Imam Ali [a], and declares his appointment by making it clear that he is the vicegerent of Muslims. The Messenger of Allah [s], on his farewell pilgrimage, when holding Imam Ali's hand told ten thousand Muslims who were present there: "Do I not have more authority over you than you on yourselves?" When they replied: "Yes, surely", then the Holy Prophet declared: "O faithful! For whomever I am his master and the authority whom he obeys: Ali will be his master. O Allah! Be friendly towards the friends of Ali; help those who help him, and hinder those who hinder him, and may the truth always be with him."
This opinion is further supported by the Messenger's sermon: "I leave two great, precious things among you, the Book of Allah and my Household (Ahlul Bayt). If you keep hold of both of them, you will never go astray after me. One of them is greater than the other the Book of Allah is like a rope hanging from heaven to earth. The other is my Family and Household. Remember these two will never be separated from each other until they encounter me at Kawther (in Paradise)."
The followers of this opinion thus see that the Imamate concerned is only Imam Ali [a] and his infallible progeny.
It is clear that an Islamic state and Muslim communities are in need of a just Islamic government led by a just (president) who sets up correct laws and preserves the interests of Muslims. Indeed, the existence of an Islamic state headed by an Imam (leader) committed to Islamic principles is not only a necessity but is also a legal obligation. It is the duty of Muslims to strive for the establishment of an Islamic state, led by a just Imam, following the principles of the Holy Qur'an and the traditions of the Prophet (sunnah) so that Muslims can live in its protective shadow of security, happiness and prosperity.
Characteristics of a Muslim Imam (leader) are:
1. He should be familiar with Islamic rules, legislation and laws.
2. He should be just, God-fearing, pious, and adhere to Islam.
3. He should be adequate to run Muslim affairs politically, socially, economically ...etc.
Adopting an Islamic system with all its obligations, needs a deep and accurate knowledge and a clear assimilation of the Holy Qur'an and the Prophetic traditions. Such cannot be achieved unless the Imam is a jurisprudent with extensive understanding of Islamic laws.
To be just is also an essential prerequisite for the Imam in order to keep away corruption and deviation.
Allah, the Almighty, says:
"And do not incline to those who are unjust, lest the Fire touches you..."
Holy Qur'an (11:113)
And also says:
"...and if any fail to judge by (the light of) what Allah has revealed, they are (no better than) the infidels."
Holy Qur'an (5:47)
 While Imam Ali [a] was in prayer in the Holy Prophet's Mosque in Madina, a man came seeking charity. No one took any notice and he was about to leave disappointed when Imam Ali [a] pointed his finger towards him while he was bowing in prayer. The man took the ring from the Imam's finger. While this was happening in the Mosque, this verse was revealed to the Prophet Muhammad [s].
 Sunan ibn Maja / Merits of Imam Ali [a]