Microscopically uniform anisotropic etching of semiconductor layers is a critical step in ME(O)MS and ULSI circuit fabrication. The non-ideal etched feature limits density, yield and reliability of these devices. Artefacts such as RIE-lag, notching, sidewall bowing, micro-trenching, and mask facetting are typically accompaniment effects occurring during the etching high aspect ratio features in silicon. Because etch rates and the shape of etched features depend on circuit layouts-design, considerable effort have to be spent in the near feature to understood all common single and simultaneous phenomena during the high aspect ratio dry etching. The development of effective manufacturing processes requires a fundamental understanding of the factors, which determine etched feature shape. Gas reactivity, pressure (affecting ion bowing in the sheath due to scattering with neutrals), ion, electrons and reactant transport to the surface, and product transport away from the surface, have been identified as the key factors that control the microscopic etching uniformity in high aspect ratio etching. The choice of these can cause numerous secondary aspect ratio-dependent effects. The modelling of the most significant effects as RIE-lag, notching, bowing, facetting, micro-trenching, profile shape dependence etc. will be discussed.