In situ studies of the magnetic fields in space by spacecraft (S/C) began 4 decades ago. The experiments were initially difficult to perform and interpret accurately because of contamination by S/C-generated magnetic fields, partial measurements of the vector field, limited mass and power for the instrumentation, low-data-rate sampling, discontinuous or spotty data coverage, and sometimes unknown orientation or location of the S/C. As each of these challenges was overcome, an initial view of the macrostructure of the interplanetary magnetic field (IMF) emerged, substantiating the Archimedean spiral structure and solar origin of the IMF as proposed theoretically by E. N. Parker. An important mesoscale feature noted early on was the sectoring of the IMF into an ordered sequence of alternating polarities associated with solar sources. Subsequent studies catalogued the microscale magnetohydrodynamic structures: shock waves, and tangential and rotational discontinuities and fluctuations. Correlations with solar photospheric magnetic fields, coronal holes and in situ plasma, and energetic particles elaborated more completely on the structure and dynamics of the IMF. This paper will provide a brief historical summary of the study of the IMF or, as it is now more commonly referred to, the heliospheric magnetic field (HMF). ¿ 2001 American Geophysical Union