The transition from galactic to extragalactic cosmic rays is discussed .
One of critical indications for transition is given by the Standard Model of Galactic cosmic rays , according to which the maximum energy of acceleration for iron nuclei is of order of E _ { Fe } ^ { max } \approx 1 \times 10 ^ { 17 } eV .
At E > E _ { Fe } ^ { max } the spectrum is predicted to be very steep and thus the Standard Model favours the transition at energy not much higher than E _ { Fe } ^ { max } .
As observations are concerned there are two signatures of transition : change of energy spectra and elongation rate ( depth of shower maximum in the atmosphere X _ { max } as function of energy ) .
Three models of transition are discussed : dip-based model , mixed composition model and ankle model .
In the latter model the transition occurs at the observed spectral feature , ankle , which starts at E _ { a } \approx 1 \times 10 ^ { 19 } eV and is characterised by change of mass compostion from galactic iron to extragalactic protons .
In the dip model the transition occures at the second knee observed at energy ( 4 - 8 ) \times 10 ^ { 17 } eV and is characterised by change of mass composition from galactic iron to extragalactic protons .
The mixed composition model describes transition at E \sim 3 \times 10 ^ { 18 } eV with mass composition changing from galactic iron to extragactic mixed composition of different nuclei .
These models are confronted with observational data on spectra and elongation rates from different experiments , including Auger .