Let J ⊂ I be ideals in a formally equidimensional local ring with λ(I/J) < ∞. Rees proved that λ(I n /J n ) is a polynomial P (I/J)(X) in n of degree at most dim R and J is a reduction of I if and only if deg P (I/J)(X) ≤ dim R − 1. We extend this result for all Noetherian filtrations of ideals in a formally equidimensional local ring and for (not necessarily Noetherian) filtrations of ideals in analytically irreducible rings. We provide certain classes of ideals such that deg P (I/J) achieves its maximal degree. On the other hand, for ideals J ⊂ I in a formally equidimensional local ring, we consider the multiplicity function e(I n /J n ) which is a polynomial in n for all large n. We explicitly determine the deg e(I n /J n ) in some special cases. For an ideal J of analytic deviation one, we give characterization of reductions in terms of deg e(I n /J n ) under some additional conditions.2010 Mathematics Subject Classification. Primary 13D40, 13A30, 13D45, 13E05. Theorem 1.9. (=Theorem 4.4) Let (R, m) be a formally equidimensional local ring of dimension d ≥ 2, J I be ideals in R and J has analytic deviation one. Suppose l(J p ) < l(J) for all prime ideals p in R such that ht p = l(J). Then the following are true.(1) If J is not a reduction of I then deg e(I n /J n ) = l(J) − 1.(2) If l(J) = d − 1, depth(R/J) > 0 and for all n ≥ 1, √ J : I = √ J n : I n then J is a reduction of I if and only if deg e(I n /J n ) ≤ l(J) − 2.We can not omit the condition depth(R/J) > 0 from Theorem 4.4 (2) (see Example 4.5). We give sufficient conditions on the ideal J for the equality √ J : I = √ J n : I n for all n ≥ 1 (see Proposition 4.3).3