The complexity of parallel systems has produced a large collection of semantics for processes. Van Glabbeek’s linear time-branching time spectrum provides a classification of most of these semantics; however, no suitable unified definitions were available. We have discovered how to unify them, both in an observational framework and in an equational framework. In this first part of our study we present the observational semantics, that stresses the differences between the simulation (branching) semantics and the extentional (linear) semantics. As a result we rediscover the classification in van Glabbeek’s spectrum and shed light on it, obtaining a framework where we can consider all the semantics in the spectrum at the same time. Also, we have discovered some “lost links” that correspond to semantics, possibly not too interesting (at the moment), that provide a clearer picture of the spectrum.