The Common Blue (Polyommatus icarus) can easily be confused with the Brown Argus (Aricia agestis).
The best way to tell the difference is to look at the underside of the wings, and more particularly the pattern of the spots.
There are two distinguishing features:
1. The Common Blue has a spot (the "cell spot") on the underside of the forewing that is absent in the Brown Argus;
2. When the butterfly is resting, the Common Blue has three dots at the top of the underside of the hindwing in a horizontal line, while with the Brown Argus the third dot closest to the head is not horizontally alligned with the other two but closer to the wing edge. We call this feature the "hockey stick".

In southern European countries the Common Blue is easily confused with the Southern Common Blue (Polyommatus celina) and the Adonis Blue (Polyommatus bellargus).
The Southern Cmmon Blue replaces the Common Blue in Southern Spain, North Africa and the Canaries.
The Common Blue, Southern Common Blue and the Adonis Blue look very similar but the Adonis Blue has a very characteristic black chequering on the white margins (fringes) of its wings, where the veins cut the margin (in Dutch: "geblokte franje").

Athens, Greece, 6 June 2016