The concept of Instrument Packages is a practical technique implemented with the ultimate goal of ensuring that the observations prepared by the users are error-free and are executed efficiently at ESO's telescopes. We present here a summary of what Instrument Packages (IPs) are, and how they are defined, built, and deployed for all La Silla and Paranal Instruments. IPs are an integral part of the interface for our observation-oriented tools like P2PP (Phase 2 Proposal Preparation) and OT (Observing Tool). In our presentation we demonstrate the central role that IPs play in the observation process -- from preparation to execution -- while being served to the users in a fully transparent way.

Instrument Packages are software bundles containing the following elements:

 * Some pre-established, parametrized observation sequences that are provided as template files by the instrument scientist of each instrument. Observations are prepared by the user in the form of Observation Blocks (OBs) with the P2PP application by assembling these instrumental templates together. They are read by P2PP and allowed values of the observation parameters are presented to the user via its Graphical User Interface.

 * Verification modules (EVM - External Verification Module) allow users to check the validity of their OBs. They can run complex checks on the users observation parameters that P2PP cannot perform on its own. These are verifications involving inter-template or inter-OB dependencies, compliance checks with values in the user's observation proposal, restrictions imposed by the observing strategy used, and other special constraints. The user is allowed to submit OBs only when the verification process does not report any errors.
 
 * Execution time calculators (ETRM - Execution Time Reporting Module) aid in calculating the total execution time of OBs and help maintain compliance with the allocated telescope time. 

 * Also, provided as a set of configuration files are instrument-specific observational constraints that are to be considered for execution in Service Mode.


New revisions of the Instrument Packages are provided to the community for every observing period, in order to reflect the most recent status of the instruments available on Paranal and La Silla telescopes. The same IPs are used both by the users to prepare their OBs and at the telescope to execute them. The update of the IPs is the result of a collective work and the modification of any part of it (templates, EVM, ETRM...) is done fully transparently for the end users. Indeed each observing program is associated with its Instrument Package in the central ESO database that is retrieved by P2PP when needed.

Thanks to all their built-in features, the Instrument Packages simplify the access to the instrument for the astronomer, vastly reduce manual verifications on ESO's side, and stop faulty OBs from reaching the Observatory. All these factors contribute to improving the observational efficiency at the telescope.