The determinants and costs of control were studied in 6 experiments examini
ng the performance costs of changing stimulus dimension (digit value/number
of elements) or attention strategies (speed/accuracy) on the first trial a
fter task transition. Costs were compared for task shift and reconsideratio
n only. preparation ability was studied by presenting all transition inform
ation at the beginning of a a-part block or only prior to each part. Result
s showed pronounced first-trial transition costs. Different factors were as
sociated with stop-start and task-switching requirements. Transition costs
were separate from those of basic task performance. Costs were sensitive to
global control considerations and were larger for task dimension changes t
han for attention strategy shifts. Costs involving task dimension change, b
ut not strategy shifts, were reduced with advanced preparation. These resul
ts are discussed in relation to contemporary models of control. A new disti
nction is proposed between activation and execution of control strategies.