A Unified Approach to Difference Sets with gcd(\u3ci\u3eV, N\u3c/i\u3e) \u3e 1

The five known families of difference sets whose parameters (v, k, λ; n) satisfy the condition gcd(v,n) \u3e 1 are the McFarland, Spence, Davis-Jedwab, Hadamard and Chen families. We survey recent work which uses recursive techniques to unify these difference set families, placing particular emphasis on examples. This unified approach has also proved useful for studying semi-regular relative difference sets and for constructing new symmetric designs