Abstract Emile and Odette Thellier produced the first reliable determinations of paleointensity following an experimental protocol used earlier by Johann Koenigsberger. Although Koenigsberger did groundbreaking work on thermoremanent magnetization (TRM), it was the Thelliers who formulated the fundamental idea of partial TRMs as building blocks for TRM. In his 1938 doctoral thesis and a series of short notes, Emile Thellier minutely examined the data on TRM and partial TRM, ultimately establishing for bricks and other baked clays his laws of pTRM reciprocity, independence and additivity. In 1946 he speculated that blocking represents “…immobilization of elementary magnetic moments below a temperature Θ … Θ will vary at each point in the body, perhaps with the dimensions and the shape of the crystalline grains … One can thus explain thermoremanence by the progressive fixing, in the course of cooling, of moments which find themselves held fast when they pass through their individual temperature Θ.” Thellier thus established the physical basis of TRM blocking and recognized the essential role of grain size and shape. In 1949 Louis Neel quantified these concepts in terms of the properties of single-domain grains. Today the Thellier–Thellier method remains the benchmark of reliable paleointensity data. The challenge has been the non-ideality of real geological and archeological materials: TRM carriers larger than single-domain size and physicochemical alteration during heating. The Thelliers avoided these problems by using bricks and pottery previously fired under conditions similar to those in laboratory heatings, eschewing volcanic and other rocks. But despite their problems, we have to deal with the material nature provides. This paper provides insights into the physics underlying the Thellier–Thellier method and check procedures that detect non-ideal behavior, as well as reviewing recent advances in paleointensity methodology.