Biological analyses traditionally probe cell ensembles in the range of 103-106 
cells, thereby completely averaging over relevant individual cell responses, 
such as differences in cell proliferation, responses to external stimuli or 
disease onset. In past years, this fact has been realized and increasing 
interest has evolved for single-cell analytical methods, which could give 
exciting new insights into genomics, proteomics, transcriptomics and systems 
biology. Microfluidic or lab-on-a-chip devices are the method of choice for 
single-cell analytical tools as they allow the integration of a variety of 
necessary process steps involved in single-cell analysis, such as selection, 
navigation, positioning or lysis of single cells as well as separation and 
detection of cellular analytes. Along with this advantageous integration, 
microfluidic devices confine single cells in compartments near their intrinsic 
volume, thus minimizing dilution effects and increasing detection sensitivity. 
This review overviews the developments and achievements of microfluidic 
single-cell analysis of intracellular compounds in the past few years, from 
proof-of-principle devices to applications demonstrating a high biological 
relevance.