Permanent magnet synchronous motors (PMSM) are gaining popularity day by day and will rule in the next decade as these motors have several benefits over induction motor such more efficiency, low noise operation, reduced size, and maintenance free operation. These motors require a position sensor for its control, which makes it costlier, increase the size and low reliability. The development of fast microcontroller, IGBT devices, and high density permanent materials has opened the gate of industrial and commercial market for PM Synchronous Machine. The researchers from various corners of the world are working hard from more than two decades to evolve the sensorless control scheme for rotor flux estimation. A sensorless control schemes proposed till date works satisfactory, but, high performance is still a challenge. The paper presents a review of most popular robust SMO technique for position sensorless control of PM-Synchronous Motor with its benefits & drawbacks.