The electrohydrodynamics of soft interfaces and the processes underlying interfacial charge formation by, for example, unsymmetrical ion adsorption are important aspects of current research on the electrosurface phenomena. In particular, the recent progress in both fields greatly benefits from the now-possible accurate evaluation and quantitative interpretation of (ionic) excess conductivities at solid surfaces and in 3D polyelectrolytic architectures. Achievements in the proper formulation of the conceptual and theoretical framework and in the improvement of measurement capabilities have been tightly connected to the work of Johannes (Hans) Lyklema (1930–2017). Considering his valuable contributions, we herein summarize the theoretical basis of surface conductivity analyses, review the experimental options for the quantification of the surface conductivity at macroscopic planar solid–liquid interfaces, and discuss exemplary surface conductivity case studies for soft and hard interfaces permeable or not to ions and fluid flow. Dedication: Johannes Lyklema (November 23, 1930–October 31, 2017) was a key scientist in colloid and interface science. He completed his doctorate at the University of Utrecht in 1957 under the supervision of Professor J. Th. G. Overbeek with a thesis entitled ‘Adsorption of counterions.’ In 1963, he was appointed a Professor of Physical and Colloid Chemistry at Wageningen Agricultural College (later named Wageningen University), a position he kept until his retirement in 1995. Under his directorate, the Laboratory of Physical and Colloid Chemistry at the Wageningen University became a world-leading research center known for its key research in the fields of interfaces, macromolecules, and electrochemistry. Johannes Lyklema published almost 400 articles and wrote the five-volume text book ‘Fundamentals of Interface and Colloid Science’; he gave lectures in five languages and in five continents, received honorary doctorates in Belgium, Sweden, and Spain, and has been awarded with the Order of the Dutch Lion, to quote only a few of the distinctions and awards he received during his career. In his research, Johannes Lyklema paid particular attention to the analysis and interpretation of the electrosurface phenomena. He was the founder and chairman of the International Advisory Board of the conference series Electrokinetic Phenomena (ELKIN) and section editor on electrokinetics in this journal (Current Opinion in Colloid and Interface Science). We will remember Johannes Lyklema as an eminent scientist as well as a warm-hearted and outstanding person.