Angular differential imaging (ADI) is a high-contrast imaging technique that reduces quasi-static speckle noise and enhances the detection of nearby companions around bright stars. The technique involves acquiring a sequence of images with an altitude/azimuth telescope while the instrument field derotator is switched off, allowing the field of view to rotate relative to the instrument. A reference PSF is constructed from other images in the sequence and subtracted to remove quasi-static PSF structure. Residual images are then rotated and combined to improve signal-to-noise ratio. ADI has been tested with Gemini North data, showing a 5-fold reduction in PSF noise per image subtraction and an additional gain of the square root of the number of images. For a one-hour sequence, ADI achieves a total speckle noise attenuation of 20-50 compared to a single 30s exposure. For Vega, a two-hour sequence achieved a PSF noise attenuation of 100, reaching a 5-sigma contrast of 20 magnitudes at 8". ADI achieves 30 times better signal-to-noise than classical techniques for 30-minute sequences. The technique can detect exoplanets with masses of ~1 M_Jup in orbits between 50-300 AU around nearby young stars. ADI can be combined with other high-contrast imaging methods. The technique is effective for a wide range of declinations and is particularly useful for detecting faint companions. ADI produces a reference PSF from the same target sequence, eliminating the need for PSF calibration with nearby stars or sky exposures. The stability of the PSF is crucial for ADI performance, as it determines the noise attenuation and the regime in which the noise is reduced with increasing observing time. Observations with Altair/NIRI at Gemini showed that the PSF evolves on timescales of ~10-60 minutes, with reference image subtraction achieving ~2-6 noise attenuation for short intervals. The technique is well-suited for detecting jovian companions with masses greater than 1-2 M_Jup at separations of 50-300 AU. ADI can be combined with other techniques like SSDI, high-order AO, and coronagraphy to improve detection limits. The research highlights the effectiveness of ADI in reducing speckle noise and detecting faint companions around bright stars.Angular differential imaging (ADI) is a high-contrast imaging technique that reduces quasi-static speckle noise and enhances the detection of nearby companions around bright stars. The technique involves acquiring a sequence of images with an altitude/azimuth telescope while the instrument field derotator is switched off, allowing the field of view to rotate relative to the instrument. A reference PSF is constructed from other images in the sequence and subtracted to remove quasi-static PSF structure. Residual images are then rotated and combined to improve signal-to-noise ratio. ADI has been tested with Gemini North data, showing a 5-fold reduction in PSF noise per image subtraction and an additional gain of the square root of the number of images. For a one-hour sequence, ADI achieves a total speckle noise attenuation of 20-50 compared to a single 30s exposure. For Vega, a two-hour sequence achieved a PSF noise attenuation of 100, reaching a 5-sigma contrast of 20 magnitudes at 8". ADI achieves 30 times better signal-to-noise than classical techniques for 30-minute sequences. The technique can detect exoplanets with masses of ~1 M_Jup in orbits between 50-300 AU around nearby young stars. ADI can be combined with other high-contrast imaging methods. The technique is effective for a wide range of declinations and is particularly useful for detecting faint companions. ADI produces a reference PSF from the same target sequence, eliminating the need for PSF calibration with nearby stars or sky exposures. The stability of the PSF is crucial for ADI performance, as it determines the noise attenuation and the regime in which the noise is reduced with increasing observing time. Observations with Altair/NIRI at Gemini showed that the PSF evolves on timescales of ~10-60 minutes, with reference image subtraction achieving ~2-6 noise attenuation for short intervals. The technique is well-suited for detecting jovian companions with masses greater than 1-2 M_Jup at separations of 50-300 AU. ADI can be combined with other techniques like SSDI, high-order AO, and coronagraphy to improve detection limits. The research highlights the effectiveness of ADI in reducing speckle noise and detecting faint companions around bright stars.