Abstract: An apparatus for use in controlling first and second downhole tools includes a first cyclical indexing mechanism associated with a first downhole tool, and a second cyclical indexing mechanism associated with a second downhole tool, wherein the first indexing mechanism defines at least three sequential indexing positions within a cycle and the second indexing mechanism defines at least two sequential indexing positions within a cycle. The apparatus includes at least one actuator for actuating the first and second indexing mechanisms in response to a common stimulus to cause said first and second indexing mechanisms to advance between respective indexing positions so as to permit co-ordination of the operational states of the associated first and second downhole tools. The apparatus may be used for controlling first and second under reamers or for controlling an under reamer and a corresponding stabilizer.