This paper describes the relationship between two types of commonly used models in control and identification theory: state-space and input-output models. The relationship between the two model structures can be explained in terms of a newly formulated set of parameters called the observer Markov parameters. This is different from the usual connection between the two model structures via well-known canonical realizations. The newly defined observer Markov parameters generalize the standard system Markov parameters by incorporating information of an associated observer. In the deterministic case, the observer Markov parameters subsume a state-space model and a deadbeat observer gain In the stochastic case, the observer Markov parameters contain information of an optimal observer such as a Kalman filter. The observer Markov parameters can be identified directly from experimental input-output data. Making use of the new connection, one can extract from the identified observer Markov parameters not only a state-space model of the system, but also an associated observer gain for use in modern state-space based feedback controller design.