Solid state fermentation (SSF), biofiltration and soil bioremediation are processes involving microorganisms grown on solid or semi-solid substrates or supports. Monitoring these processes requires the measurement of environmental parameters (temperature, pH, water content and activity) and the carbon cycle (biomass, substrate concentration, CO2). However, given the complexity and heterogeneity of the solid medium, these variables are not easily accessible and measurable. This paper reviews current on-line methods and innovative applications of methods with a potential to measure parameters in SSF. Direct measurements of temperature, pH, and water content are considered employing classical sensors, and indirect measurements of the biomass by respirometry or pressure drop (PD). More recent methods such as: aroma sensing, infrared spectrometry, artificial vision, and tomographic techniques (X-rays, Magnetic Resonance Imaging or MRI) are then explored. (C) 2002 Elsevier Science Ltd. All rights reserved.