We present an extensive study of the structure and energetics of monovacancies and antisites in InP. Using a first-principles approach, the different charge states of indium and phosphorus vacancies and antisites are examined. The lattice distortions around the defects are derived fully self-consistently with respect to both electronic and ionic degrees of freedom. Jahn-Teller relaxations, defect-induced one-electron energy levels, and ionization potentials in the band gap are discussed. From the formation energies we predict the favored vacancies and antisites under different stoichiometry conditions.