All of the discussions in earlier chapters have assumed that each of these licenses can be and will be enforced by their licensors, and, ultimately, by the courts. However, two unique problems (in addition to those involved in the enforcement of any contract) affect licensors of software under open source and free software licenses.
First, for each license described in previous chapters, the licensor may not even know who the licensees are. All of these licenses, to varying degrees, put forth the licensed code with an invitation to adopt it and use it, subject to the terms of the respective licenses. These open source and free software licenses do not require notification or other affirmative action to be taken by licensees that would notify the licensor of the fact that the licensee has entered into the contract. In addition, most of these licenses permit and even encourage the free sublicensing of the licensor's work to other licensees, whose connection to the original licensee can become tenuous as the licensed work moves through multiple generations of licensing before ending up with a particular user.
 The SCSL, which is not an open source or free software license, although it incorporates some of their principles, does require some form of notification. The Microsoft Shared Source Initiative operates under totally different custom-negotiation principles, so they know who they are dealing with from the outset.
Second, while some of these licenses require that the licensee engage in some affirmative action to access the licensed work (such as clicking on a button indicating that the licensee agrees to be bound by the terms of the license) prior to permitting access of the licensed work, many of them like the BSD, MIT, and Apache Licenses do not. Others, like the GPL and LGPL, do not require such affirmative assent in all cases.
Both of these problems are substantially addressed by the fact that use of the licensed work is contingent on accepting the terms of the license. Unlike other types of contracts, open source and free software contracts impose very few, if any, affirmative obligations (such as the payment of royalties) on licensees, but rather impose restrictions only on the rights granted by the license. This property will operate, most likely, to save the enforceability of these licenses from challenges regarding the absence of mutual consent or consideration that may otherwise arise.