Cell membrane association by several important peripheral proteins, such as Src, MARCKS, HIV-1 Gag, and K-Ras, requires nonspecific electrostatic interactions between a cluster of basic residues on the protein and acidic phospholipids in the plasma membrane. A simple theoretical model based on the nonlinear Poisson-Boltzmann equation describes well the experimentally measured electrostatic association between such proteins and the cell membrane.