Various materials that show reversible changes in the rheological properties in response to external stimuli, especially electric and/or magnetic fields, have attracted attention because of their possible applications to devices of transformation of electric/magnetic stimulus into mechanical (passive) force. These materials are called electrorheological (ER), magnetorheological (MR), and electromagnetorheological (EMR) materials. The field-responsive properties are regarded as rheological functions induced by the field, which characterize these materials. The ER/MR materials can be classified into some types, i.e. suspensions, homogeneous or heterogeneous liquids, elastomers/gels, etc. according to the rheological characteristics in the absence and presence of the electric/magnetic fields. For each type, the field-induced rheological properties are briefly summarized, followed by some topics on the material preparation, stimuli-induced functions, and/or practical applications are exemplified. The materials chemistry, especially for organic/polymer materials, are mainly focused on here.