Messier 101 (M101, NGC 5457) was discovered by Pierre Méchain on March 27, 1781, and added as one of the last entries in Charles Messier's catalog. It was one of the first "spiral nebula" identified as such, in 1851 by William Parsons, the third Earl of Rosse.

Although extended 22 arc minutes on photos and quite bright, only the central region of this galaxy is visible in smaller telescopes, best at low powers. Suggestions of the spiral arms can be glimpsed in telescopes starting from 4 inch as nebulous patches. Several of these patches (i.e., spiral arm fragments) were assigned their own catalog numbers by William Herschel and later observers; according to the NGC and Burnham, there are 9 such numbers, 3 of which go back to Herschel who has found them on April 14, 1789, while the RNGC states that five of the others don't exist (ne); it mentions however that deVaucouleurs has them as knots: NGC 5447 (H III.787), 5449 (ne), 5450 (ne), 5451 (ne), 5453 (ne), 5455, 5458 (ne), 5461 (H III.788), 5462 (H III.789), and 5471.

On photographs, however, the Pinwheel Galaxy M101 is revealed as one of the most prominent Grand Design spirals in the sky. While quite symmetric visually and in very short exposures which show only the central region, it is of remarkable unsymmetry, its core being considerably displaced from the center of the disk. Halton Arp has included M101 as No. 26 in his Catalogue of Peculiar Galaxies as a "Spiral with One Heavy Arm".