Nucleophilicity:

The incoming nucleophiles' relative nucleophilic strengths will influence the rate of the S_N2 reaction along with stronger nucleophiles reacting much faster. A charged nucleophile is stronger as compared to the corresponding uncharged nucleophile (for example alkoxide ions are stronger nucleophiles than as compared to alcohols). Nucleophilicity is as well related to base strength while the nucleophilic atom is similar (for example RO^- > HO^- > RCO₂^- > ROH > H₂O). The order of nucleophilic strength for the halides is F^- > Cl^- > Br^- > I^-, in polar aprotic solvents.

Because the rate of the S_N1 reaction is not dependent of the incoming nucleophile, the nucleophilicity of the incoming nucleophile is not important.