We investigate how various invariants of elliptic curves, such as the
discriminant, Kodaira type, Tamagawa number and real and complex periods,
change under an isogeny of prime degree p. For elliptic curves over l-adic
fields, the classification is almost complete (the exception is wild
potentially supersingular reduction when l=p), and is summarised in a table.Comment: 22 pages, final version, to appear in Trans. Amer. Math. So
