The present issue, Volume 3a of "Boron Compounds" 4th Supplement of the Gmelin Hand- book, presents the description of boron nitride and a part of other boron compounds containing nitrogen. Volume 3 b (to be published Later) will complete the presentation of the boron-nitrogen compounds and will also cover boron compounds containing fluorine. Due to technical circumstances, Volume 1 (systems with hydrogen) and Volume 2 (systems with oxygen) of this particular supplement will be published subsequently, whereas Volume 4 (boron compounds containing CL, Br, I, S, Se, and Te, as well as a section containing carboranes) has already been published. ALL volumes of the 4th Supplement will be augmented by a formula index. The IUPAC nomenclature is generally adhered to; occasional abbreviations for compounds are explained in the text. A positive sign for the chemical shifts of the NMR signals indicates a 1 13 downfield shift from the references, usually internal (CH ) Si for 6 H and 6 C and external 3 4 11 (C H h0-BF for 6 B, others being specified. 2 5 3 In contrast to more recent publications but remaining consistent with the previous volumes of the Boron Series, the iminoborane structure is written as B=N and not as B=N. In fact, the real electronic structure is more realistically symbolized by B=N, but it seems reasonable to avoid =NR in accordance with the commonly accepted "iminoborane" nomenclature.