The Solar and Heliospheric Observatory (SOHO) is a joint ESA/NASA mission to study the Sun, from its deep core to the outer corona, and the solar wind. SOHO was launched on 2 December 1995 and was inserted into a halo orbit around the L1 Lagrangian point on 14 February 1996. From this vantage point it is continuously monitoring the Sun, the heliosphere, and the solar wind particles that stream toward the Earth. Nominal operations of the SOHO mission started in April 1996 after commissioning of the spacecraft and the scientific payload. Detailed descriptions of the twelve instruments, which represent the most comprehensive set of solar and heliospheric instruments ever developed and placed on the same platform, can be found in The SOHO Mission (Solar Physics, Vol. 162, Nos. 1-2, 1995). §This volume contains papers reporting the first scientific results from the SOHO mission as well as descriptions of the in-flight performance of some of the instruments, published in two parts of a Solar Physics special (Part I in Solar Physics, Vol. 170, No. 1; Part II in Solar Physics, Vol. 175, No. 2). Unique data from the three helioseismology instruments (GOLF, VIRGO, MDI/SOI) provide new insights into the structure and dynamics of the solar interior, from the deep core to the outermost layers of the convection zone. The remote sensing instruments (SUMER, CDS, EIT, LASCO, UVCS, SWAN) present exciting new data on a wide range of topics such as transition region dynamics, coronal plumes, coronal holes, streamers, and coronal mass ejections, giving us our first comprehensive view of the outer solar atmosphere and corona. These data are complemented by energetic particle measurements produced by the ERNE instrument on board SOHO.
