An Outline of the History of Restriction on Ijtihad
This group took recourse in him for guidance in all religious matters in all their detail, learning and committing to writing the precepts and teachings of their Imam who had been designated by God, the Exalted, and was preserved against any kind of error. Thus they recorded from each Imam, infallible and designated by the preceding imam, one after another, up to the Master of the Age in occultation, may God's blessings be upon all of them.
This group of Muslims adhered to the thaqalayn (lit. 'two precious things') from God, the Exalted, which their prophet (s) had left behind for his ummah. These two are, the Book of God and the Household of the Prophet (s), which 'will not separate from each other until they meet him (s) at the Pool' , as has been narrated in a large number of traditions recorded by both Sunni and the Shi'i traditionists.  The motto of this group is 'following' (tashayyu'), because they follow 'Ali, may peace be upon him, and the Imams of his descent, relying on none except the Ahl al-Bayt, may peace be upon them, unaffiliated with any of the schools of fiqh.
They are known as "Ja'fari" not because Ja'far ibn Muhammad, may peace be upon him, is an imam of only this school, but because during his lifetime the ruling regimes, that of the Umayyads and the `Abbassids, were weak and preoccupied with their own problems, and this gave the Shi'ah a respite which they used fully to acquire the knowledge of the ahkam (laws) and other teachings from their Imam. Thus the propagation of the madhhab (school of law) of the Ahl al-Bayt and widening of the circle of its influence during the lifetime of Imam Abu `Abd Allah Ja'far ibn Muhammad al-Sadiq, may peace be upon him, resulted in the Shi'ah being identified by ,his name, though the qualification "Ja'fari" is sometimes also used to distinguish them from Zaydi Shi`is.
Summarily, the Shi'ah do not submit to anything whatsoever except the Book of God, the Exalted, and the Sunnah of His prophet as received from the Imams of his Household, who are secure from error and lapse. This has been their custom since the time of the demise of the Prophet, may God bless him and his Household, to the present day. They act as per the literary meanings of the Holy Qur'an and its unambiguous verses (muhkamat), and refer the verses standing in need of interpretation (mutashabihat) to the Imams. They follow the Sunnah of the Prophet (s) as narrated through reliable chains of transmission from the Infallible Household and compiled in books and the `usul' (early collections of Shi`i hadith) which are extant to this day either in their original form or with their contents rearranged in chapters, as explained in detail in the introduction of our book al-Dhari'ah. 
[The Continuity, Meaning, and Legal Significance of Ijtihad in the Imami Shi'i Tradition:]
The Shi`i ulama', in all the eras, have applied ijtihad for understanding the literal meanings of the Holy Qur'an and the Sunnah, in the sense that they have been deriving religious laws from these two sources by applying specific rules of deduction. However, in the early period the prerequisites of ijtihad were few and its method was simple, and it was possible for even ordinary people, to say nothing of the learned and the elect, to attain the knowledge of divine laws by practising ijtihad.
But later, with the passage of time and after the era of the Imams and the beginning of the ghaybah (Occultation), and the vicissitudes faced by the books and the usul and their compilers, with the dispersion of manuscripts in different parts of the world along with its natural consequences in the form of differences resulting from discrepancies and lapses committed due to error or forgetfulness of copyists and editors, despite their credibility and care in recording-all this contributed to the proliferation of the prerequisites of ijtihad. Its acquisition came to require, the study of a number of sciences relevant to understanding the texts and literal meanings of the Qur'an and the Sunnah and to determining the authenticity of the narrators and the chains of transmission for distinguishing the authentic from the faulty and the praiseworthy from the blameworthy, among other things.
Moreover, the practice of ijtihad in the sense of making full effort for deriving the rules of the Shari'ah and determining the import of the texts and the credibility of chains of transmission of traditions-is the obligation of every capable individual (wajib'ayni) in the opinion of all Shi'i scholars and if an adequate number of persons do not meet it, all other mukallaf individuals are obliged to fulfil it. But if others are performing this duty to an adequate extent, the rest are relieved of the, obligation.
That which we have mentioned regarding the consensus of Shi'i 'Ulama' concerning the obligation of ijtihad in deducing religious laws is true of, their actual practice, though some later scholars have verbally disavowed it on the claim of acting according to the traditions (akhbar). They are therefore known as Akhbaris. But we have explained elsewhere that this is only a verbal dispute, because one cannot act in accordance with traditions without acting according to their meanings as understood and interpreted. Hence acting on a tradition depends on comprehending its meaning and purport, and we do not imply by ijtihad anything except the derivation of the meanings of traditions and deduction therefrom,? and this is something common to all Shi'i 'ulama'.