Electronic light microscopy involves the combination of microscopic te
chniques with electronic imaging and digital image processing, resulti
ng in dramatic improvements in image quality and ease of quantitative
analysis. In this review, after a brief definition of digital images a
nd a discussion of the sampling requirements for the accurate digital
recording of optical images. I discuss the three most important imagin
g modalities in electronic light microscopy - video-enhanced contrast
microscopy, digital fluorescence microscopy and confocal scanning micr
oscopy - considering their capabilities, their applications, and recen
t developments that will increase their potential. Video-enhanced cont
rast microscopy permits the clear visualisation and real-time dynamic
recording of minute objects such as microtubules, vesicles and colloid
al gold particles, an order of magnitude smaller than the resolution l
imit of the light microscope. It has revolutionised the study of cellu
lar motility, and permits the quantitative tracking of organelles and
gold-labelled membrane bound proteins. In combination with the techniq
ue of optical trapping (optical tweezers), it permits exquisitely sens
itive force and distance measurements to be made on motor proteins. Di
gital fluorescence microscopy enables low-light-level imaging of fluor
escently labelled specimens. Recent progress has involved improvements
in cameras, fluorescent probes and fluorescent filter sets, particula
rly multiple bandpass dichroic mirrors, and developments in multiparam
eter imaging, which is becoming particularly important for in situ hyb
ridisation studies and automated image cytometry, fluorescence ratio i
maging, and time-resolved fluorescence. As software improves and small
computers become more powerful, computational techniques for out-of-f
ocus blur deconvolution and image restoration are becoming increasingl
y important. Confocal microscopy permits convenient, high-resolution,
non-invasive, blur-free optical sectioning and 3D image acquisition, b
ut suffers from a number of limitations. I discuss advances in confoca
l techniques that address the problems of temporal resolution, spheric
al and chromatic aberration, wavelength flexibility and cross-talk bet
ween fluorescent channels, and describe new optics to enhance axial re
solution and the use of two-photon excitation to reduce photobleaching
. Finally, I consider the desirability of establishing a digital image
database, the BioImage database, which would permit the archival stor
age of, and public Internet access to, multidimensional image data fro
m all forms of biological microscopy. Submission of images to the BioI
mage database would be made in coordination with the scientific public
ation of research results based upon these data. In the context of ele
ctronic light microscopy, this would be particularly useful for three-
dimensional images of cellular structure and video sequences of dynami
c cellular processes, which are otherwise hard to communicate. However
, it has the wider significance of allowing correlative studies on dat
a obtained from many different microscopies and from sequence and crys
tallographic investigations. It also opens the door to interactive hyp
ermedia access to the multidimensional image data, and multimedia publ
ishing ventures based upon this.