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To pursue M.Tech in IITs you must appear for Graduate Aptitude Test in Engineering (GATE) conducted by an IIT each year. The eligibility criteria is that a candidate must have passed B.E/  B.Tech to get admission to M.Tech. Candidates who are studying in final year and pre final year of the qualifying examination are also eligible to apply for GATE. GATE score is valid for 3 years. You need to score minimum cut off marks in order to qualify in GATE. Each year the cut off changes depending on total number of candidates appeared in a particular discipline, difficulty level of examination and total number of seats available etc. To check the list of institutes accepting GATE Score click on the link below

https://engineering.careers360.com/colleges/list-of-engineering-colleges-in-india-accepting-gate