I. Star Formation in T and Ob Associations

o The OB ass ciati ns, which are f ssil star f rmati n regi ns, retain the endpr ducts f the star f rmati n pr cess in an pen, un bscured envir nment. Observati ns f the l w-mass stars in OB ass ciati ns pr vide a far clearer picture f the results f the star f rmati n pr cess than d bservati ns f embedded regi ns f ng ing star f rmati n. We review the X-ray and ptical surveys f the f ssil star f rmati n regi ns in the Ori n OB1 ass ciati n. L w-mass pre-mainsequence stars n t nly ab und in the kn wn regi ns f recent star f rmati n but are als distributed ver a much larger v lume. The pre-main-sequence stars have a narr w age spread, and the mass functi n extends d wn t substellar masses. The clustering f pre-main-sequence stars ar und Ori may represent an ev lved versi n f the Ori n Nebula Cluster. We speculate ab ut the effect f the OB envir nment n the initial mass functi n and the f rmati n f planetary systems like the s lar system.