An explosive booster assembly (70) which includes a primary module (10) with a first housing (12) and a first booster (14), wherein a detonator (62) is engageable with the first housing, and an auxiliary module (30, 30A) which comprises a second housing (32) with a second booster composition (34), wherein the second housing (32) is interengageable with the first housing (12) to expose the first booster composition (14) to the second composition (34), and wherein any number of substantially identical auxiliary modules (30, 30A) are engageable with one another in order to form a compound booster assembly.