Let K be a discrete valuation field with ring of integers O_{K}.  Let /pi be a uniformizing element of O_{K}. Let E be an elliptic curve over K. Let X be the minimal regular model of E over O_{K}.  For any natural integer n, let X_{n}=X x Spec O_{K}/(/pi^{n+1}).
I will present the following two topics:
1. The isomorphism class of X_{0} as a curve over the residue field k=O_{K}/(/pi). (finer than Kodaira-Neron classification)
2. The relation between X_{n} and X'_{n+m} where m>0 and X' is the minimal regular model of E over the ring  O_{L} of integers of a finite galois extension L/K.