This paper begins with a historical report of the discovery of Ca4REO(BO3)(3) (RECOB), a new family of nonlinear optic (NLO) materials. It presents the crystal growth and some physical characteristics related to their use in NLO applications. Then the NLO properties of GdCOB and YCOB are reviewed and the influence of some cationic substitutions is described. (C) 2002 Elsevier Science B.V. All rights reserved.