Abstract In situ observations are defined as those made at the point where the instrument is located. Since a large variety of techniques have been used for in situ measurements, the general features, capabilities and problems of these techniques are reviewed. To illustrate capabilities and problems more specifically, examples of recent measurements are presented which have a bearing on the chlorofluoromethane—ozone problem . These include: (1) resonance fluorescence for the measurement of Cl and CIO, (2) grab and cryogenic collection of whole air samples for the m easurem ent of CFCl3 and CF2Cl2 (as well as CH4, H2, CO and N2O ) , (3) im pregnated filters for acid chloride, and (4) matrix isolation for HO2 and NO2.