This research entitled: ―Assessment of the causes of deviation from Islamic practices among the contemporary Muslims of the North-West Nigeria was motivated by the proliferation of uncompromising groups among Muslims, each with its peculiar forms of deviation. It is an attempt to assess the causes of deviation among Muslims in the North-West Nigeria. The study therefore, adopted the survey research design. Two self-structured questionnaires formulated based on the research objectives and questions which guided the study. The first sets of questionnaires were responded to by Muslims irrespective of their religious ideologies while the second questionnaire were responded base on Muslim grouping. The Data was analysed using frequencies, percentages and tables. It revealed that failure to abide by the religious instructions lead to disbelief, innovations, transgression and immoderation among the Muslims; some Muslims perceived the Islamic rulings on some activities based on the inclinations of their various religious groups; and that the major factors causing deviation are ignorance of the objectives of shari‘ah and forsaking the practice and guidance of the best three generations of Muslims identified by the Prophet (SAW). The major effects of deviations in religious practices Include disunity, misrepresentation of Islam, threat to security and clashes and unrests among Muslims. In view of the findings, the research recommends that the solutions to deviation in religious practices are tenable only in the adherence to the Qur‘an and the authentic Sunnah of the Prophet Muhammed (SAW) as understood and practice by the Sahabah (RA). Furthermore, Muslims should accept and respect all religious evidences regardless of their religious inclination.