Answer
$$\dfrac{\pi}{2} \space \text{radians}$$
Work Step by Step
Multiply by $\dfrac{\pi\space \text{radians}}{180^\circ}$ to convert from degrees to radians:
$$90^\circ \times \dfrac{\pi\space \text{radians}}{180^\circ}= \dfrac{\pi}{2} \space \text{radians}$$
