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El libro se propone abordar un instrumento que suscita gran interés en el derecho contractual contemporáneo: el otorgamiento de facultades de ejercicio unilateral, en cuanto atiende la necesidad de hacer más ágil la contratación, de adaptar el negocio a circunstancias sobrevenidas que alteren el equilibrio del contrato originalmente convenido, de istrar los riesgos, de flexibilizar en general las condiciones del negocio acercándolo con mayor precisión a las necesidades de las partes y de los mercados en los que estos se desarrollan, e incluso a las características del tipo negocial elegido, cuya plena realización puede comportar la necesidad de cierta libertad en el manejo de los elementos estructurales del mismo. No obstante su legitimidad, el ejercicio unilateral de facultades conferidas a una de las partes comporta innegables riesgos de abuso, por lo que el objeto de la investigación que desarrolla el presente libro se centra en establecer bajo qué criterios una de las partes podría ejercer legítimamente tales facultades. A tal efecto, se proponen criterios que delimiten dicho ejercicio, acudiendo a los valores que subyacen en el principio de buena fe, a las enseñanzas de la tradición romanista y a las experiencias que, sobre la base de la utilización de estas cláusulas por diversos sectores de la economía, se han venido acuñando. Todo esto toma en consideración que este ius variandi posee una repercusión enorme sobre la entera categoría del contrato; v. gr., la determinación y modificación de la prestación, el objeto del contrato, los riesgos sobrevenidos, el resquebrajamiento del consentimiento bilateral, los límites a la responsabilidad, la preservación de la igualdad de las partes contratantes.