Pipeline bursting, production lines shut down, frenzy traffic, trains confrontation, nuclear reactor shut down, disrupted electric supply, interrupted oxygen supply in ICU - these catastrophic events could result because of an erroneous SCADA system/ Industrial Control System(ICS). SCADA systems have become an essential part of automated control and monitoring of many of the Critical Infrastructures (CI). Modern SCADA systems have evolved from standalone systems into sophisticated complex, open systems, connected to the Internet. This geographically distributed modern SCADA system is vulnerable to threats and cyber attacks. In this paper, we first review the SCADA system architectures that have been proposed/implemented followed by attacks on such systems to understand and highlight the evolving security needs for SCADA systems. A short investigation of the current state of intrusion detection techniques in SCADA systems is done , followed by a brief study of testbeds for SCADA systems. The cloud and Internet of things (IoT) based SCADA systems are studied by analysing the architecture of modern SCADA systems. This review paper ends by highlighting the critical research problems that need to be resolved to close the gaps in the security of SCADA systems.