The Small Tortoiseshell (Aglais urticae) looks quite similar to the Large Tortoiseshell (Nymphalis polychloros) which latter is quite rare in northern and western Europe.
The main difference between the two is that the Small Totoiseshell usually has 2 small spots and 1 bigger spot in the middle of its forewing while the Large Tortoiseshell usually has 4 black spots in the middle of its forewing.
Another usually reliable difference between the two is that the base of the hindwing of the Small Tortoiseshell is dark while the base of the hindwing of the Large Tortoiseshell is orange.

In eastern parts of Europe this species can also be confused with the Yellow-legged Tortoiseshell (Nymphalis xanthomelas).
The main difference with this species are the Yellow-legged Tortoiseshell having much brighter and redder colouring, enlarged blue markings on the hindwings, a clear and broad brown band on the inside of the blue markings and yellowish/brownish legs (instead of black legs).

1st photo: 's Graveland, The Netherlands, 11 July 2011
2nd & 3rd photo: 's Graveland, The Netherlands, 13 August 2012
4th photo: Eemnesserpolder, Eemnes, The Netherlands, 4 August 2014
5th photo: Oostvaardersplassen, The Netherlands, 28 June 2016