Anti addition (groups add to opposite faces):

• X2 (Br2, Cl2): Via cyclic halonium ion intermediate that blocks one face
• Example: cis-2-butene + Br2 → (2R,3R) and (2S,3S) dibromide (meso NOT formed)

Syn addition (groups add to same face):

• H2/Pd,Pt,Ni: Both H's delivered from catalyst surface to same face
• BH3 (hydroboration): Concerted 4-center TS, both B and H add from same face
• OsO4: Cyclic osmate ester delivers both OH groups syn
• KMnO4 (cold, dilute): Similar cyclic intermediate, syn dihydroxylation

Not stereospecific:

• HX addition: Planar carbocation intermediate allows nucleophile from either face
• Gives racemic mixture from achiral starting material