<p>Poikilosis is a recently introduced unifying theory in biology. It is based on the pervasive heterogeneity of all biological molecules, complexes, systems, networks, cells, organisms, and others. In a relaxed situation, such entities, called levels, are in an energetically favorable state called lagom. These states are maintained actively and passively by TARAR (tolerance, avoidance, repair, attenuation, resistance) countermeasures. Lagom is an emergent property that originates from multiple effects in a complex system. Lagom-changing effects include level extent changing processes, level conversion processes, level property changes, and temporal processes. When measuring, one should consider whether a system or entity has the lagom extent of heterogeneity, where there is a certain degree of variation, but where the measurements are equal and thus analyses should be based on intervals. Here, the concept of lagom is discussed and elaborated in biological systems, measurements, data analysis, prediction, and simulation. Special attention is given to the significance in medicine. Lagom reconstitution therapy can reduce the overall burden of diseases when curative care is not available. When a system has a non-lagom extent, the effect can spread due to the connectivity of levels. In extreme cases, such as comorbidities, there is a systemic effect, which can lead to the destruction of the system and death in the case of organisms. Consideration of lagom is essential to facilitate reliable studies and interpretations, as well as to enhance the reproducibility of experiments and studies.</p>