OSIRIS is the newest instrument at Keck and is designed to dissect  
small patches of the sky in unprecedented detail. It has now been  
used with the Keck LGS AO system for almost three years and has made  
ground breaking observations in a wide range of fields from Jovian  
moons to high redshift galaxies. It is unique in its use of a lenslet  
array to achieve diffraction limited sampling of the sky. I  
describe several science programs but focus on our own search for  
extrasolar planets and our search for the earliest disk galaxies.  
Based on its success, we are now developing a lenslet based IFS for  
the Gemini Planet Imager and as part of the IRIS instrument for TMT.