We study four processes responsible for the dewetting onset of thin (thickness < 100 nm) polystyrene (PS) films spin cast on nonwettable silicon (Si) surfaces. (i) Spontaneous dewetting is initiated by indentlike disturbances formed at the free PS surface, upon heating above the glass transition temperature (Tg) of PS. Indents become steeper as they grow toward the Si substrate, resulting in an increase of the Laplace pressure which opposes dewetting. (ii) Nanoindentation-induced surface defects either grow toward the substrate, similar to the spontaneously formed indents, or heal, resulting in a flat film surface, depending on their curvatures. Artificial indents with curvatures comparable to the ones measured for the spontaneous indents grow. Healing indents correspond to higher curvatures. (iii) Particles initiate dewetting as they sink inside the film and toward the substrate. Performing our experiments in a controlled clean room environment, we found that ∼23% of the developed dry patches were formed because of particle presence. (iv) Indents into the substrate prior to spin coating affect the uniformity of deposited PS films. The developed surface irregularities can also act as nucleation sites for dewetting initiation, upon heating above Tg.