The lack of investments in transmission and the inductive nature of the network make reactive power of key importance in system operations and planning. Moreover, the many sources of uncertainty require reserves for the reliable operation of the power system. Reactive reserves can be viewed from the load's and the generation's perspective. From the load's perspective, the focus is on the margins to collapse, a key metric in voltage stability studies. From the generation's perspective, the focus is on the amount and value of reserves, a key need in system operations and planning, and in electricity markets. This paper defines reactive reserves as seen from the load's and generation's perspectives. The computational methods available for reactive reserves are discussed and compared. The heavy nonlinear dependence of reactive reserves on the network's and the generators' physical characteristics, the plausible contingencies, the direction of load growth and the generation dispatch is shown using numerical examples.