Sources of root‐mean‐square (RMS) emittance growth are described for negative ion extractors in both volume and surface negative ion sources and plasma low‐energy beam transport systems. For surface negative ion sources of the Lawrence Berkeley Laboratory or Los Alamos National Laboratory [on Los Alamos Meson Particle Facility (LAMPF)], attention is paid to the nonlinear transverse emittance growth mechanism of the beam/warm plasma interaction. In some cases this is a large effect. In addition, non‐normal sheath fields at a convertor are examined as a source of emittance growth. For volume sources, attention is paid to aberration production due to field penetration from the extractor, RMS emittance growth enhancement by a negatively biased plasma extraction electrode, and emittance growth caused by transverse extraction across a magnetic field. For both volume and surface sources, RMS emittance growth due to nonlinear aberration at the plasma extraction electrode is analyzed. Time‐dependent contributions to emittance growth are also examined. Nonlinear saturation of ion acoustic waves caused by ion extraction from a warm plasma and beam transversal through a plasma are studied.