"Recognizing a disease process on a histopathologic slide becomes instantaneous, with increasing familiarity. Breaking this process down into the "how" is difficult, especially given that the steps may not be the same for each individual. Nonetheless, on a basic level, it is important to separate a solitary growth ("tumor" or "lesion") from a rash ("inflammatory" process, focus on the most obvious pathologic finding, and run through a differential diagnosis. With experience, that "obvious" pathologic finding (i.e., where to start) becomes second nature. The diseases in this atlas are grouped, arbitrarily, by such findings (see the Index by Pattern). Notably, basic algorithms are ultimately overly simplistic, and there is overlap of the two major divisions in Figure 1 (tumor versus rash). For example, clear cell acanthoma can architecturally mimic psoriasis, mycosis fungoides can appear to be a dermatitis, and epithelioid sarcoma can be confused with a palisading granulomatous process"-- |