The single-cell juxtacellular recording–labeling technique makes it possible to label the neuron recorded extracellularly. It is a very useful tool for achieving single-cell structure–function correlation studies in living, intact neural networks and for determining their phenotype and genotype. It can reveal the overall picture of the smallest neurons, including interneurons. It can be combined with other electrophysiological techniques (e.g. electro-encephalographic recordings and intracerebral electrical stimulation), electron microscope, immunohistochemical, molecular and/or genetic techniques. Its principle consists in iontophoresing tracer molecules into the membrane of the neuron being recorded. This is done under continuous electrophysiological monitoring, allowing the retrieval of the neuron labeled in more than 85% of attempts. Continuous DC recordings suggest that the juxtacellular filling “or tickling” procedure produces a transient micro-electroporation, which allows the internalization of the tracer into the intracellular space. Since this procedure allows single neurons to be recorded for long periods, many electrophysiological features can be collected, and the finest and remotest axonal ramifications can be marked. In spite of some limitations and pitfalls, the juxtacellular technique remains the high standard for investigating the genetic, molecular, physiological, and architectural bases of cell–cell communication. It is also a very versatile and useful tool when it comes to decipher, for instance, the molecular, cellular, and network mechanisms of brain state, physiological, and pathological oscillations.