The purpose of this paper is to provide a brief summary of the various analytical predictive criteria in order for "strange phenomena" to occur in a class of softening nonlinear oscillators, oscillators which may exhibit escape from a potential well. Implications of Melnikov's criteria are discussed first and transient chaos in the twin-well potential oscillator is illustrated. Three different heuristic criteria for steady state chaos or escape solution, proposes by F. Moon, G. Schmidt and W. Szemplifiskia-Stupnicka, are then presented and compared to computer simulation results.