Two novel in situ clearance and impact force estimation approaches, one based on a describing function approach and the other on an optimization approach, are developed for a harmonically excited vibroimpact system exhibiting a periodic motion. The estimates obtained using data from the mechanical analog of an impact oscillator and simulation using a digital computer were shown to agree with actual clearances and measured impact forces. Difficulties encountered due to unavoidable broadband noise in physical experiments are discussed and remedies suggested. The effect of input amplitude and frequency and clearance stop asymmetry due to non-identical stiffnesses and clearances on estimates were also studied.