Measurements of trace gases and photolysis rates in the troposphere are essential for understanding photochemical smog and global environmental change. Chemical measurement techniques have progressed enormously since the first regular observations of tropospheric ozone in the 19th century. In contrast, by the 1940s spectroscopic measurements were already of a quality that would have allowed the use of modern analysis techniques to reduce interference between gases, although such techniques were not applied at the time. Today, chemical and spectroscopic techniques complement each other on a wide range of platforms. The boundaries between spectroscopic techniques will retreat as more Fourier transform spectrometers are used at visible wavelengths and as wide-band lidars are extended, and combining chemical techniques will allow detection of more trace gases with better sensitivity. Other future developments will focus on smaller, lighter instruments to take advantage of new platforms such as unmanned aircraft and to improve the effectiveness of urban sampling.