Control loop oscillations are the most prominent indications of deteriorated controller performance. In many cases, a pre-dominant oscillation thwarts the original intention of feedback control. The feedback mechanism may increase the variability of the control error rather than keeping the actual process variable close to the desired target. Oscillations often indicate a more severe problem than irregular variability increase and hence requires more advanced maintenance than simple controller re-tuning. Nowadays, the detection of oscillations is a solved problem. Diagnosis of oscillations, however, still is an area of active research. Fortunately, during the last years, significant progress has been made in this direction, even though some questions remain unanswered. Nonlinearity analysis, as reported in this book has enabled important differential diagnoses mainly for oscillating loops and will be presented for stiction detection lateron. Still partly unsolved is the problem of diagnosing arising oscillations due to linear coupling between two - well-tuned, if seen individually - control loops