In order to study simple extensions of valued fields, notions of key polynomials were developed. Model theoretical properies of extensions of valued fields were also studied. The properties of valuations used in model theory shed a new light on key polynomials and they make it possible to obtain underlying properties of these extensions. Key polynomials are used for defining separate valuations which approximate a valuation on an extension K(χ). A valuation ν λ on K(χ) is separate if there is a K-basis B λ of K[ χ ] such that ν λ is determined by its restrictions to K and B λ. For every valuation ν the aim is to find a family of monic polynomials of K[ χ ], which are called key polynomials, and a family ν λ of separate valuations such that for every λ the elements of B λ are products of key polynomials, and, for every f ∈ K[ χ ], ν(f) is the maximum of the family (ν λ (f)). The approach of the present paper shows the links between some properties of valuations used in model theory and the key polynomials. The existence of a family of separate valuations as above follows in a natural way. Our definitions rely on euclidean division of polynomials, on bases of vector spaces and on classical properties of valuations.