单层扁平上皮是由形成一连续表面之扁平、形状不规则的细胞所组成，其可被称做人行道状上皮、单层鳞状上皮细胞这个名词是将细胞比喻成鱼的鳞片般。与所有的上皮一样，这种纤细的内衬由下方之基底膜所支持，如简图所示。基底膜的厚度鲜少厚到可用一般光学显微镜观察到。

单层扁平上皮位在参与气体（如肺臓）或液体（如微血管的壁）被动运输（扩散）的内榇表面。单层扁平上皮也会形成胸膜、心包膜和腹膜腔的纤细内衬，它们在此可使体液进出这些体腔中。虽然这些细胞为单层，但其具有多种重要的功能。这些细胞通常具有特化的表面受体以控制局部作用之化学传讯者的分泌。

A simple squamous epithelium is a single layer of flat cells in contact with the basal lamina (one of the two layers of the basement membrane) of the epithelium. This type of epithelium is often permeable and occurs where small molecules pass quickly through membranes via filtration or diffusion. Simple squamous epithelia are found in capillaries, alveoli, glomeruli, and other tissues where rapid diffusion is required. Cells are flat with flattened/rounded nucleus. It is also called pavement epithelium due to its tile-like appearance. This epithelium is associated with filtration and diffusion.This tissue is extremely thin, and forms a delicate lining.