We review adaptive optics (AO) in biological imaging using direct wavefront measurement. Here light from a point source in the specimen is used to measure the wavefront with a detector such as a Shack-Hartmann wavefront sensor, similar to the approach that is used in astronomy. The benefit of direct wavefront measurement relative to the sensorless methods is that the wavefront can be measured quickly in one step. Typically sensorless methods are iterative, requiring a number of measurements. Taking multiple measurements can take more time and may expose the sample to more light which can lead to photo-bleaching. Another benefit is that some indirect methods use optimization of a merit function such as image sharpness or image intensity. In direct wavefront sensing the wavefront aberration is directly measured and corrected rather than optimized. As we shall discuss, a common metric for direct wavefront measurement and correction is the Strehl ratio which is defined as the ratio of the on-axis beam intensity to the diffraction limited beam intensity. The objective is getting as close as possible to the diffraction limit.